How is sushi supposed to be eaten?

Clean your hands before you eat. Often, you will be given a hot towel for the purpose. Return the towel to the server neatly folded.

It is fine to eat sushi with your hand, as long as there is rice with each piece of fish. Sashimi (sliced fish not prepared with rice) should only be eaten with chopsticks.

If you sit at the sushi bar, feel free to converse with the itamae (sushi chef) if he's not too busy, and to order sushi from him. Do not order anything but sushi from him, order non-sushi items from your server. It is classy to ask the itamae to recommend a sushi item or even to ask him to surprise you. If there is something you know you don't like (uni is mine), let him know because it is very rude to not clean your plate. By that I mean don't even leave a grain of rice.

Mixing wasabi with the soy sauce is so common that you can get away with it, but it is considered kind of tacky, at least in a fine restaurant. So is eating ginger with a bite of sushi. You're correct that it is intended as a palate cleanser between sushi 'courses'. Again, if you want to, you're not going to get kicked out for it. In fine sushi restaurants, sushi is often not served with wasabi on the side. After all, the chef is an expert, he knows more about sushi than you! And presumably he knows how much wasabi should go on each piece of sushi. If you really want more, go ahead and add it if it's there, or let the itamae know your preferences in the matter.

Regarding dipping into the soy, try not to soak the rice of nigiri-zushi (shaped rice with a slice of fish on top). Just lightly dip the fish. Ideally, place the bite into your mouth fish down. Try to eat the whole piece in one bite. If you must, bite it in half but don't put the second half back on the plate. It's funny, but the video in @Mel's comment to the question shows tossing a big piece of sushi down pretty well :) Just know that the dunking it in soy sauce is a joke!

If you are in a part of the word where tipping is expected, tip the server and itamae separately. Often there will be a jar on the bar for tipping the chef. He would rather not touch money while he is working.

You can offer the itamae a sake if you're so inclined.

Finally, leave your chopsticks laying across the soya bowl between uses, or on the holder if there is one. Never leave them on your plate or (heaven forbid) sticking out of your rice.

Regarding your edit, not necessarily. It is completely optional to use chopsticks to eat rolls or nigiri. It is entirely up to you. It's like using the provided spoon to eat the usually provided miso soup. Drinking it from the bowl is totally acceptable.

It sounds like you were served sushi rolls as finger food, and if chopsticks are not provided, then you are expected to eat it like the other finger food, with your fingers. There isn't really etiquette involved as this isn't a restaurant setting. If you are having a sit-down meal, and there are no chopsticks provided, you could always ask for a pair.

Typically in Japan's fine dining sushi restaurants, sushi maki (the rolls) are not served, only sushi nigiri (individual portions,) and neither wasabi nor soy sauce is provided to its patrons. The chef will add the wasabi he/she believes is the correct amount for the piece of sushi, usually placing it between the bed of rice and the piece of fish/squid/scallop/etc. The chef will also brush down the fish/squid/scallop/etc. with the appropriate soy sauce mixture (it's a blend of soy sauce and other stuff) he/she believes is the correct amount. So all you really have to do is pop it in your mouth. Chopsticks or hands are actually fine, just make sure you pop the entire thing in your mouth, don't bite it in half.

Wasabi is also only good when grated fresh. The chef will use a special wasabi grater as he/she is serving your sushi. Most likely, you are thinking of green-dyed horseradish. My recommendation isn't to eat it, but feel free to try it however you like.

The pickled ginger is used as a palate cleanser, yes, it's between fish. Your sushi should be served in a particular order, but failing that, it should still be eaten in a particular order. You start from the lightest, and work your way to the heaviest items, in terms of flavour. This order is something like: scallop -> squid -> white fish (e.g. ????, hiramasa, kingfish) -> red fish (e.g. ???, maguro, tuna) -> fatty red fish (e.g. ??????, maguro ootoro, the fatty belly of the tuna.)

If you are in a restaurant that provides soy sauce, dip your chopsticks into the soy sauce and brush it onto the fish/squid/scallop side (don't dip the rice side into the sauce, it soaks up way too much and it just tastes like soy sauce.) If you are eating sushi maki, brush it onto the cut side of the piece.

An info-graphic I found on the topic a little while ago that I believe answers your question quite well: Source: iLoveCoffee.JP

In terms of etiquette is it alright to eat sushi by hand? If relevant I'm in North America.

According to the info-graphic there is no wrong way to eat sushi as long as it makes it into your belly. I would agree with this however I would recommend at least attempting to use the chopsticks.

It's my understanding the ginger is to reset the taste pallet so you should eat a little before switching to a different roll.

That's it!

but if you're provided with chopsticks, you should use them, right?

Again you should at least attempt to. It's not a bad thing to learn :)

I have never seen anyone eat sushi with their hands in the U.S., and I basically live on a sushi diet. That said, if everyone else were doing it and if the sushi tasted good enough, I would totally eat sushi with my hands.

Re: wasabi. Put it in the soy sauce. Eat ginger between pieces if you want to.