From Eat This, Not That! The Best (& Worst) Foods in America!:

Sushi 101
There are dozens of different ways to enjoy sushi, but if you want to be a true zen master of raw fish consumption there are rules that are worth knowing and following. Avoid another sashimi-gobbling faux pas by brushing up on some basic sushi etiquette.

• When you're not using your chopsticks, place them parallel to each other in the holder or on the dish. Never stick your chopsticks into your rice and leave them sticking up.
• When passing food from one plate to another, use the opposite end of your chopsticks to make the delivery, not the part you eat with.
• You can eat sushi rolls and nigiri with your hands, but you should eat sashimi with chopsticks. And forks should never enter the picture; sushi chefs don't want their creations tainted with the metallic tang of silverware.
• Eat sushi in one bite, especially sashimi and nigiri. The flavors and textures come in small packages for a reason - so that you can experience them all at once. There are a few exceptions, especially in the United States, where specialty rolls can be as big as cannons; you'll know an exception when you see it.
• The sliced ginger is for cleaning your palate in between different tastes, not for draping all over the delicate fish before you.
• As fun as it might be to evoke the nasal-clearing effects of wasabi overdose, that pile of concentrated Japanese horseradish is supposed to be used sparingly - if at all. A good sushi chef applies the correct amount of wasabi to his creations, meaning all you need to do is be ready to deliver them from plate to mouth.
• Rice-making is a craft that takes sushi chefs years to master, so don't go mucking up their perfect grains by drowning them in soy. if you want to dip, dip the fish side of the sushi into the sauce, not the rice side. The rice will soak up the soy like a sponge, compromising flavor and texture and leaving you with a surplus of sodium in your belly.