My main bad habits would be biting my nails and procrastination. I tend to bite my nails without thinking if I'm nervous and/or bored; it's one habit I've had forever, unfortunately. As for procrastion...eh. Doesn't help that I get overconfident about being able to finish things, which is why I tend to put them off until the last moments...I would think I would have learned by now...
Actually, I guess another habit would be that I stay up way too late; I have a tendency to do that a lot.