Here’s a nice tutorial on how to politely use your chopsticks. It includes some items I didn’t know, such as:
When you finished [sic] using disposable chopsticks (wari-bashi), put them back in the bag and fold the bag in half. This indicates that they are used.
It also has a couple of things I did know, including the following taboos:
Sticking chopsticks vertically into a bowl of rice.
Passing food from your chopsticks to somebody else’s chopsticks.
The site doesn’t mention why these are bad, but I already know. These things are done only at funerals. I learned about the first one from Todd when I was in Japan. The second one I read about here (and mentioned here). Suffice it to say that the imagery of the second one is nasty enough that anyone would want to avoid doing it.