When you're in the middle of a roleplay, DON'T DO THIS. It is rude.
That counts for multi-clienting, too. Unless you've absolutely got to or you have lightning fingers, don't do it.
Don't watch movies while roleplaying. (I used to do this and come back from a long scene to find the player I was with logged off.)
Especially don't play other games while roleplaying.
Now, admit it. Who does this? (I know who all of you are... Don't try hiding.)