How do you keep turkey from getting too dry?

I suggest using a brine. Lots of brining details can be found in the answers to this question:

What are the basics of brining meat?

Alton Brown has a really great recipe for brined roasted turkey. You can watch the Good Eats excerpt on YouTube in which he covers brining turkey.

I also suggest reading The Basics of Brining (PDF) from Cook's Illustrated.

In addition to what has been mentioned above, roast breast side down for all but the last 20 minutes or so. Most of the fat is in the back, and will essentially self-baste. Flip the bird (heh) towards the end of cooking to crisp up the skin.

Lay a few pieces of bacon across the turkey while cooking. This essentially makes it self-basting as well.

Edit by @SamtheBrand: See here for a recipe.

Follow-up edit by Scivitri: Roasting a turkey is a complex, although usually fairly forgiving process. There are many, many recipes online, and all recipes are best followed as a loose guideline. Personally, I combine ideas from family practice, several recipes, how I feel at the moment, and then cook it to an appropriate temperature. So my original suggestion for applying bacon (which I took from family practice) was meant as something which could be done to almost ANY method you use when roasting a turkey.

For example, this year we wanted to try brining a turkey. I started from Alton Brown's recipe, but applied bacon rather than canola oil. I also turned the temperature down a bit further for roasting, and roasted the bird longer (and to a bit higher temperature). It still came out lovely.

Anyway, I didn't want my general suggestion to turn into a narrow suggestion to follow a specific recipe (which I have not used). Sam's recipe may be great; but the idea is more general.

I also recommend cooking the bird in a bag. It helps keep in all of the natural juices of the turkey, and you have to supply less of your own.

The first time I ever cooked a turkey, we used one of these. The turkey came out perfectly. We also injected the bird with copious amounts of garlic herb butter under the skin.

The reason a turkey comes out dry (in the breast meat anyway) is because turkeys have two kinds of meat on them (leg and thigh v's breast) that like to be cooked in different ways. The dark meat of the leg and thigh likes long slow cooking whereas the breast likes hot fast cooking. You can do all sorts of fiddly things but removing the legs and cooking them separately then adding the breast (still on the bone if you like) later. You can then assemble the pieces to look like a whole turkey on a platter, all perfectly cooked! In the UK we don't have thanksgiving and many people now opt for a goose at Christmas rather than a turkey, for the reason they have more flavour and don't get so dry.

Perhaps the biggest thing for me: don't overcook it. The best tool you have available to you is a probe thermometer, preferably one with a temperature alarm.

Dark meat is done at a higher temperature than white meat is, and if you cook at one temperature until the legs are done, the breast will be parched. I'm a big fan of the Alton Brown method: in addition to brining, start the turkey at high temperature (500F) for half an hour or so, then drop the temperature to 350 and take the bird out when a thermometer in the breast reads 161F. It's come out perfect every time for me.

This site recommends three steps: cook it breast-side down for most of the time, don't overcook, and use chicken stock in the bottom of the pan and for basting.

Another technique is to brine your turkey, or use a kosher one. (Using a kosher bird allows you to skip the brining step.) I've never really tried non-kosher turkey, but here are some references from Cook's Illustrated that talk about why they like kosher turkey/chicken over non-kosher.

An illustrative quote: "Appearance aside, perhaps the most noticeable difference between the winning bird and the others we sampled was that the winning bird tasted juicy and well-seasoned. To remove as many impurities as possible,the chickens are buried in salt for one hour and then rinsed off with cold spring water. The combination of salt and water acts like a brine, encouraging the fibers in the meat to open and trap the salt and water, leading to a juicier, more flavorful bird." From this article.

A similar discussion about chicken breasts.

Brining is good. Basting is also good. Martha Stewart does this thing where she covers the turkey with cheesecloth, and bastes the cheesecloth with a mixture of wine and butter (a bottle and a pound, respectively), and that works better than you'd ever believe. The cheesecloth comes out looking black and disgusting, and the turkey underneath looks like you cut it out of a magazine.

In the end though, the best thing you can do is cook it the correct amount of time. I'd cook it to ~165 degrees (unstuffed), even though the gov't usually says something like 180 degrees. 180 is way too high. Remember also, that the internal temperature of the bird will continue to increase for a while after you take it out of the oven, so take it out when it's still a bit below your desired temperature.

Stuff under the breast skin with a sausage meat based stuffing, rub the whole bird with butter and then lay bacon over the breast. See:

http://www.deliaonline.com/recipes/cuisine/european/english/traditional-roast-turkey-with-pork-sage-and-onion-stuffing.html

You have to weigh the bird again and recalculate time before cooking if you stuff a bird (I wouldn't recommend stuffing the cavity as it seems to slow cooking too much)

Here's an oddball that I heard about on NPR (food scientist Harold McGee (someone should tell him about this site!)): ice packs on the breasts for one hour before cooking.

Sounds weird? Yeah I thought so too. I've cooked about 6 turkeys in my time and this method yielded by far the best breast meat.

Why? Because there are two kinds of meat. So you want them to cook at different rates. There are a few methods to do this, but the ice pack is the only method I've heard of that leaves a nice looking intact bird.

Place one ice pack on each breast and hold them in place with gauze wraps. Let the turkey come up to room temp. Then proceed to cook normally. I tested this out this Thanksgiving and it resulted in breast meat that was much more succulent. The breast meat started out at 42 F and ended at 160 F while the thigh meat started at 54 F and ended at 172 F. The important thing here is that the temperature difference that you started with is the temperature difference you end with.

See my answer here: Does brining a chicken/turkey before roasting really make a difference?