I have one suggestion that might not help a 13 year old -- get your tongue pierced.
I've bit my nails my whole life and the only time I was successful at stopping was after I had my tongue pierced and the 4 years or so I still had the piercing. Sadly, I got in a habit of removing it occasionally and it ended up growing closed and I'm now back to biting my nails.
Getting fake nails would help too, but the chemicals involved have me reluctant to try it again. Would love to give the manicure a try, but my nails are always too short to get anything done to them. Sigh, guess I need to go back to the piercing shop.
