I don't have any habits like compulsively biting. In fact, I can't stand chewing gum, no matter the flavour or brand. It feels (and tastes) like plastic to me..
But, I do have the habit, when nervous or tense, of fiddling with my hair.
Of course, anyone else plays with my hair, and they're setting themselves up to be slapped 
MY SO will compulsively bounce his leg up and down as Xandax describes. That drives me totally nuts, and it will set my own nerves on edge.