P. - I FEEL YOUR PAIN!!!
I was never a nail biter, but as a teenager I was a cuticle picker. I think it stemmed from boredom or nerves for me.
I have 3 children and my oldest (12) is a horrendous nail AND toe nail biter - It disgusts me to even write that uugghh.
She has had - green puss - infections in her big toes before
and STILL cannot manage to stop. It used to drive ME insane. I tried punishing her, slapping her hand when near her mouth, the bitter tasting nail stuff, bandaids, taping cotton gloves to her hands, SHE has even taped socks over her hands at night - nothing works - NOTHING. We have tried major rewards from getting a kitten to MP3 players to special trips just for her. She just cannot stop herself, it is an unconsious habit especially when watching TV, or in bed at night. The only time she eases up is when playing the DS or on the computer.
Now I did make her a diversion when she was about 7 or 8. I discovered that she was fantastic at undoing knots in shoe laces for everybody. I make her a "Picking Toy", with her name on it. All I did was find random pieces of ribbon, a split ring (for keys) and 2 small squares of fabric and fabric paint. Cut all the ribbon to similar lengths, bunch them up at the top, sew the 2 squares together encasing the ends of the ribbon and make a loop to attach the split ring. Then I just knotted and braided all the ribbons into a "nest" of knots and gave it to her to pick at while she watched TV etc. I worked well for a while, now she feels she is too big for it.
The sad thing is "she" says she wants to stop, and she really tries from time to time. I have basically decided not to pick this battle (excuse the pun) as bigger ones are always around the corner.
Good luck
M.