Is there something I can do to keep ice-cream soft?

Yes, actually, there are two magic ingredients: Guar gum and Xanthan gum.

Guar gum is a thickener, but in small quantities can also prevent the growth of ice crystals which would cause the ice cream to harden into icicles.

Xanthan gum is a stabilizer which helps keep air (called overrun) in the mixture. Air is generally churned into ice cream by ice cream machines, but it won't stay that way without the stabilizer.

The magical ingredients for commercial ice cream are stabilizers, emulsifiers, and really good freezers. As Aaronut notes, stabilizers can go a long way... Personally, watching a bowl of ice cream melt without losing its shape makes me a bit uncomfortable, so... use in moderation.

But if you don't happen to have any gum available, here are a few suggestions drawn from my personal experience with home churning:

You want air. Lots of air.

My little (1.5qt) churn came with a bunch of recipes starting out at 2/3rds of the final volume (1qt). That's enough if I want to serve it within a few hours, but since I don't have a blast freezer in my kitchen the end result tends to lose some air while hardening. I've found that aiming for a post-churn mixture where air is around 50% of the volume works much better.

Start with a custard

Yes, I mean eggs. Egg yolk. There's some additional fat in this, but you're working with cream so you should have plenty of that already. There are also emulsifiers and proteins, and I suspect this is where the real value comes in: remember, the eventual goal is to end up with a sort of frozen foam stable enough to resist falling while hardening. It's also nice if you're able to mix in that air without turning the milk fat into butter... I aim for a maximum temperature of 140° to 160° F when cooking the custard, as this seems to provide sufficient texture without curdling (but if you do have problems with the mixture curdling, try using a double boiler). The final product should be thick enough to coat the back of a spoon, and you should chill it as quickly as possible (and you want it good and cold before trying to churn it - at least down to 40° F).

Use plenty of sugar

This is your anti-freeze. It won't keep ice crystals from forming, but it will keep the liquid from freezing solid (the more crystals form, the more concentrated the solution and the lower the freezing temperature). If you add the sugar to your custard, you can be sure it's properly dissolved - sugar granules don't do you much good. Once frozen your perception of the sweetness will decrease, so if you're tasting as you go along don't be afraid to go a bit beyond what you'd normally be comfortable eating.

Use heavy cream

Well, this should be a given, but... The lower the milkfat content, the more water and hence more ice you'll end up with. You can combat this with more sugar, but you'll still have trouble whipping in enough air because you'll have less fat to stabilize it.

Harden it fast, store it cold

You probably don't have a way to blast-freeze the final product either, but you can still do your best: make sure your deep freeze is as cold as possible (I keep mine at or below -10° F), put the ice cream into small, thin-walled containers (I re-use pint-size yogurt containers) and bury them in frozen veggies. Using multiple containers has the advantage of letting you take one out to consume without exposing the rest to room temperature, but more importantly it increases surface area: if you must put it all in a single container, try to find a wide, shallow one.

And once you have it cold, keep it cold - the longer it takes to freeze, the bigger the ice crystals will be, but the colder you keep it once frozen the less they'll grow over time (you'll also lose less air if you can avoid freeze/rethaw cyles). Not so much of an issue if you want to eat it tomorrow, but critical if you're aiming for soft creamy goodness a week or two out. Avoid auto-defrosting freezers for this same reason.

Experiment!

There are a lot of variables here. Fat content, sugar, other ingredients, the design of your churn and temperature of your freezer, ambient temperature, altitude, container size, personal taste... Don't be afraid to play around with things until you hit a recipe and process that you're happy with! It took me a couple of days to get comfortable making vanilla ice cream, but several months of trial and error (wonderful, delicious trial and error...) to get a pumpkin ice cream I was happy with. Fortunately, it's fairly easy to find folks willing to help eat your "mistakes"...

Both sugar and alcohol lower the freezing point of water and keep the ice cream "softer" at lower temperatures. Too much sugar and the ice cream is too sweet. A little bit of alcohol goes a long way in lowering the freezing point though.

One of my favourite ice cream recipes is Whisky and Honey Ice Cream. 2-3 tablespoons of whisky in a half gallon batch of honey ice cream adds great flavour and ensures that I can open the freezer and enjoy a spoonful of ice cream immediately.

Here's my recipe: http://www.triplemotion.com/2008/12/26/whisky-honey-ice-cream/

I don't like emulsifiers or other chemicals. In my experience ice cream with no additives will stay nice for a week in the freezer, if it's lasting longer than that, it's probably something wrong with the flavour!

You're getting a lot of somewhat misleading answers. Stabilization and emulsification are important topics, but to deal with excessive hardness you have to suppress the freezing point. Pastry chefs and commercial ice cream makers do this almost exclusively by varying the balance of sugars.

Sugars are water-soluble and have relatively low molecular weights, which gives them a high degree of freezing point suppression. The easy answer here is to add more sugar. It will work, but you may end up with something too sweet for your tastes. Luckily, there are sugars besides table sugar (sucrose), some of which are both less sweet and offer greater freezing point suppression. The ideal choice is dextrose, which is just granulated glucose. It's about 70% as sweet as sucrose and offers twice the freezing point suppression. you can buy on Amazon, or some health food stores or supplement stores* You can vary the proportions of sucrose and glucose to get the sweetness and the freezing point where you want.

Another good trick is trimoline, also called invert syrup. You can buy at a cake and pastry supply store, or make your own (refer to internet). This is a thick syrup that's half glucose and half fructose. It's about 20% sweeter than sucrose, and offers significantly more freezing point suppression. I like to start with a sugar blend that's 10% or 15% trimoline by weight, because it offers some stabilizing and texture improving properties in addition to controlling freezing point.

The second most common way to supress freezing point is with alcohol. Despite what you might read from some sources (David Lebovitz) including alcohol will not make your ice cream smoother. It will usually do the opposite, since most sources of alcohol are also sources of water. So I recommend using alcohol only if you're making an alcohol flavor (rum, etc.). Then you know it's going to drop the freezing point, and you can compensate with your sugar mix.

Yes, there's math involved here. Or excessive trial and error. Pick one!

*Do not confuse with "atomized glucose," which is a powder made from dried glucose syrup. It seems similar, but has very different properties, including much less freezing point suppression.

I'm using a Krups ice cream maker. Without easy access to E numbers, and only a domestic ice cream maker, I find that the biggest variables are fat, sugar and water content.

Without an expensive ice cream maker, you can't really rely on air or flash freezing. My maker just won't stay that cold longer enough, and it can't churn enough air into the mix.

The biggest improvements to my own ice cream came from perfecting my custard and just not using flavourings that add too much water. And before churning, I try and get the custard as cold as possible. Usually I leave it in the fridge over night.

The key is to keep your ice crystals small. Freezing it fast (as mentioned by Knives) is one option. I've seen liquid nitrogen and dry ice each advocated to this end. The other option is additives.

Commercial ice creams are quite the chemistry set indeed. The gums mentioned as well as methylcellulose and carageenan form gels and minimize ice crystal formation. Glycerol monostearate and lecithin both emulsify and limit ice crystal formation.

Technically Xanthan Gum is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris. It is not associated with milk ingredients, but was discovered on corn. Many people believe that those with corn allergies should avoid this ingredient, however commerically produced it is generally corn free.

Adding a dash to your ice cream will definitely improve ice crystal formation, thickening and hardening. However, the real secret is fat content. Fat doesn't freeze, but water does. So, if your ice cream is mostly milk it has a relatively low fat content and will turn out much harder. Use heavy whipping cream to make ice cream that you prefer to stay softer.

The main "not so secret ingredient" is sugar. My second "not so secret ingredient" is pectin (non-secret swiped from, IIRC, the side of a Hagen-Daz® sorbet container on the list of ingredients.) I'm a lot more comfortable with pectin than weird stuff that only shows up in processed foods, and pectin is on the shelf at most any grocery store.

I simply mix in some powdered pectin - I keep a box in the kitchen for that purpose, with the inner bag wrapped up tightly after it's first opened. Some folks like to dump in a jar of jelly but I find that blows my budget and does not seem to work any better. If cost is no object or the only good/easy way you can get a flavor you want is in jelly or jam, go there.

Not being a person who likes to watch things over and over again, I did have a script of a Good Eats (Alton Brown) ice cream episode floating around the kitchen for reference and insight for quite some time. But the basics are ~30% sugar and some pectin, IME.

It would be all about controlling the water and preventing ice formation assuming we are dealing with ice cream and not sorbets or ices. Any of the common ingredients added somehow bind free water and prevent it from reaching out and joining forces with other water to form crystals.

Extra solids in the form of milk powder or even cream cheese are particularly effective at this as well as egg yolks.

Solids are probably the single most effective ingredient that makes ice cream smooth and prevent crystallization. Sugar is number two. Corn syrup is used because it contributes a certain texture while being less sweet tasting than table sugar so we can add a touch more without making the ice cream taste too sweet.

With extra solids and extra sugars we will get exceptional texture but there will still be free water so we soak that up with micro sponges like guar gum. No one would make a gravy without a starch so why do people always expect ice cream makers to thicken ice cream without a starch? Guar gum's microparticles puff up like tiny sponges and now being little sponges instead of water droplets water cannot bump into other. Water droplets form big drops of water and finally ice crystals.

Lastly, you could boil the milk for 4 minutes and skip everything except the extra solids. That is called denaturing. That is what Hagen Daz does. The milk protein unwinds and when it reforms it traps water in its newly formed strands.

I have been making ice cream for 24 years.

I suggest grain alcohol as a possible "magic ingredient" (this time backed by experience.) I had made a ice cream of sorts with low-fat yogurt and sucralose. Left in the freezer overnight, it became too hard to scoop.

I let it thaw in the refrigerator (about a quart), then added two tablespoons of Everclear grain alcohol (95% alcohol, 190 proof). I also added a tablespoon of corn starch, cooked for a minute in half a cup of milk. I then ran the whole thing through my countertop ice cream maker. It now retains a nice, soft texture after freezing.

I'm very late to this party, but no one has mentioned marshmallows. The other answers are better and what the OP was probably looking for, but marshmallows can be used to great effect in ice cream. They provide the fluffiness you get from other methods. I can't find the recipe I really like, but here are three other examples, but ones I haven't tried: this, this, or this.

This is our more-or-less canonical question on ice cream softness, and it mentions a ton of things to pay attention to - but if you have trouble with too hard ice cream, it is hard to decide in which order to troubleshoot. My answer repeats many of the points made in other answers - but the added value is that you can decide what you should try next. While it is not impossible to try a later point before you have taken care of an earlier one, you will probably get the most mileage out of your quest if you follow the order here.

1. Use proper ratios

This is the first place I'd start: your recipe. While it is true that, if everything else is just right, you can produce creamy ice cream with outlandish recipes, I would strongly suggest that you first learn how to get a good result with a benign recipe before you start exploring new territory.

McGee's "On Food and Cooking" suggests that dairy-only recipes need around 17% fat and 15% sugar to be smooth, and recipes that include a stabilizer of some kind (egg yolk for French style, starch for gelato style, or emulsifying additives) can get away with less, but still typically over 10% fat. Try using recipes from a reliable source without changing them first, and check the percentages against this suggestion.

2. Use an ice cream maker

There are ways to freeze ice cream base without an ice cream maker, and they work in a pinch, but the resulting texture is not very good. I am not saying you should never use them, they can be fun, or the best option available - but if you want to optimize your texture, this is the most basic prerequisite. Hand-cranked ice cream makers work, of course - you just have to be willing to do the cranking.

3. Use the proper temperature

Your ice cream maker has to be at the coldest temperature to which you can get it, and the base has to be as close to 0 C as possible. If the maker is the freezer-bowl type, freeze it for much longer than it says in the manual, preferably at least 48 h. When you cool down the base, don't just let it go down to fridge temperature (4 C in the USA, frequently higher in Europe) - use an ice bath to chill it down to almost 0 C.

4. Use emulsifiers

There is a huge variety of options to choose from, some already mentioned in the other answers. I won't get too deep into the difference, there are books on that, and the choice can be quite subjective, depending on what texture you like and what you can easily buy. Note that they are good, but not a cure-all: if adding a reasonable amount (use tested recipes!) is not sufficient for you, then adding more of the same, or a different one, won't necessarily make it better, but rather give you a gummy texture and reduce the aroma.

5. Make your ice cream with more overrun

If you can change the speed of your machine, or use a different dasher, try it. These options are rarely available though, so your only option on this front might be to use another machine which has been designed for higher speed.

6. Depress the freezing point.

The typical methods are to add alcohol and/or salt, and to replace part of the sugar with a sweetener high in fructose. An answer here also mentions propylene glycol.

7. Use a better machine

There is quite a bit of difference between ice cream machines, even within the same type. This is one case where you get what you pay for - the more expensive machines tend to make smoother and softer ice cream. Of course, the correlation is not perfect, so reading comparison reviews is recommended.

8. Store the ice cream at higher temperatures before serving

Maybe you got a book by a famous ice cream parlor and followed the recipes to the T, even bought the kind of machine they recommend, and hoped to get the exact same texture as in the parlor - but when you took it out of the freezer, it was harder than expected. The difference here is that ice cream parlors' vitrines are optimized for serving temperature, not for storage. If you desire more softness after executing all the previous points, you will have to let the ice cream warm up a little bit before serving - ideally, you can let it spend some time in a * ice cube drawer, but if that's not available, the fridge is also a good option. You'll have to measure your optimal time, it will depend on a lot of variables including the recipe you used, the container you are storing it in, and how much ice cream is left in the container.

A professional ice cream machine -- one that doesn't have a base which lives in your freezer, is going to be the best way.

As @Ray said -- you need to keep the ice crystals as small as possible. The best way to do this is to start with an extremely cold base and freeze it as quickly as possible.

Premium commercial ice creams don't have to contain stabilizers and gums since they're made on machinery designed to freeze as quickly as possible.

That being said, I've had luck with this recipe: http://www.nytimes.com/2007/08/01/dining/01mini.html?_r=1&oref=slogin

Which does not use crazy chemicals just cornstarch.

My Mum always made ice cream with carnation milk, once it started to set she would stir it well every hour or so to froth it up again. I have a batch in the freezerat the moment I hope it tastes like the good old days.. Kate

I made peach ice cream that stayed soft in the freezer. Half gallon recipe: 2 cups peaches (sliced and peeled; blanch the whole peaches for easy peel) 3/4 cup sugar 1 Tbsp. lemon juice 1 tsp cinnamon (optional) --Add these and rest in fridge for 2-8 hours --puree half of this and add to pan with 2 cups halfnhalf 1 cup heavy cream 1/4 cup sugar 1/2 cup brown sugar (or more or less for sweetness) --add 4 beaten egg yolks and vanilla extract then cook to make custard --refrigerate to cool then make ice cream; halfway through process, mash other half of peaches then add.

The softening could be from the egg yolks (custard), the peach juice, or the alcohol from the vanilla (I use mexico vanilla from st luis; has the red rooster "sello de calidad". It is delicious but requires more volume which is more alcohol)

It took about an hour until the motor stopped. That is long but my machine is a tiny space saving bucket and the addition of the mashed peaches probably warmed the mixture.

Even better try glycerine, a form of clear liquid sugar, don't forget to deduct the sugar value of the glycerine from the recipe. It always works for me.