Since nailbiting tends to be a subconscious habit, it might help to use some coating on your nails that tastes awful to provide some negative feedback when you do it. I know we use Bitter Apple to train our dogs not to chew things. I am sure that pharmacies probably have something like that for human use to discourage thumbsucking and nailbiting. I know its not as damaging a habit as smoking and hopefully a whole lot easier to break.

My parents tried that on me when I was a kid. It had a lot of cayenne pepper in it. I think it was called Thum. But it did not work. I did not mind the taste (and like stuff like After Death and Dave's Gourmet Insanity sauces to this day), but it hurt a hell of a lot when I stuck my fingers in my eyes.

My little friend's parents have tried some stuff on her fingernails, but it does not work either. I do not know what is in it.