I got out of my nail biting habit by keeping my nails cut pretty short, that way there isnt much to bite. After you get in the habit of keeping your nails cut.. its a lot easier to break your nail biting habit.
Try putting another habit in place of the nail biting. When I was 13, I tried painting my nails, and then instead of biting my nails, I would pick off the nail polish. It worked (but made me look fidgety, I'm thinking). You could also try smooth "worry" stone...you rub the stone rather than bite your nails.