I think like a lot of what others have already mentioned, I do the same expression my characters' are doing, especially when its an expression I really want to get down right. This only usually only happens for more "negative expressions" involving frowning, sad faces, uncertainty etc. If my characters are smiling, I usually don't smile xD
Another bad habit I get is the more I get into drawing, my face gets closer and closer to the paper. Though I think this may have to do with bad posture instead ^^;