This is serious, people. No wisecracks
As maître of Chopsticks, I shall list a few things you should never do with chopsticks in the presence of Chinese people. Why? To be polite, cultured and not to act like a buffoon. This mini guide assumes that you know how to use chopsticks and that the obvious will not be listed (such as picking your nose with chopsticks).
1. Never use them as drumsticks, never separate chopsticks from one hand.
You should never do this, because the Chinese believe that everything that is good comes in twos. If you separate the chopsticks, you'll disturb the "peace."
2. When setting down your chopsticks, never stick them into your food.
If you stick them in the food so the chopsticks stand up, it will represent your tombstone! Death = bad! I know that this is the same in Japanese culture. If you must set them down, set them down on the table or sideways on your bowl.
3. If one single chopstick is dirtied.
A continuation of the first rule. If for some reason you can't use one of the chopsticks from your pair, get another pair, don't get a single other chopstick.
4. Never point chopsticks at someone.
Especially at your elders, you should never point chopsticks at someone, or even worse speaking with a mouthful of rice. It is impolite.
5. Don't stick your finger.
A continuation of rule #4, if you do not use chopsticks the "default/proper"* way, make sure one of your fingers (especially index) does not stick out. It is impolite.
*There is the default way of using chopsticks, which makes you look really suave. Funnily enough, this is the way foreigners learn how to use chopsticks. I, and most of my family and friends, don't use chopsticks the "default/proper" way. For example, the correct way urges you to articulate (only) your fingers, whereas I (the incorrect way) use the skin between my thumb and index to control the chopsticks. In other words, the "proper" way vs. the "natural" way, as MrFurious puts it.