Never grab food from another person’s chop-sticks using your own chop-sticks
There is a custom at Japanese funerals where the bones of the deceased are passed from one person to the next using chop-sticks and then finally being placed into the urn. Passing food from chop-stick to chop-stick is viewed to be in bad taste.

Never ‘plant’ your chop-sticks into your bowl of rice (i.e. so that it looks like rabbit ears)

Again, this harkens back to a Japanese tradition where food is often offered to the ancestors and placed into the rice like so.

One bite!
Sushi, when eaten with either hands or chop-sticks, should be consumed with one bite. Never nibble. One bite! Hikeeba! If the futomaki is too big for your mouth, then do not order it.

Too much soy sauce, a baka gaijin makes.
I still struggle with this one. When eating sushi, one should be careful not to place a standing puddle of soy sauce in the bottom of your given dish. Instead the amount shold be somewhere between a one-atom-thickness stain on the dish itself and the aforementioned puddle (closer to the former).

Don’t destroy your rice
Japanese rice is made to be sticky. This makes it perfect for eating with chop-sticks. However, it is considered bad form to let your rice soak up so much soy sauce that it looses the very property that makes it loved.

Bookends

When you are about to eat: itadakimasu (ee-ta-da-key-mas)
When you are doing eating: gochisosamadeshita (go-chi-so-sa-ma-de-shta)

-m