Well this may sound odd, but I took St. John's Wort for several months and found that I just stopped without thinking about it. One day, I was like "oh, wth? I have fingernails?"
After deciding it didn't do much depression wise, I moved to Serzone, which also didn't do anything, and Wellbutrin, which also didn't do anything. Interestingly(?), I also resumed biting my nails.
Totally anecdotal, and even I'm not sure it was the St. John's Wort, but you could give it a shot (if you're not already on other medication).
Also, if you make a habit of grooming your nails, you will probably do better, if for no other reason than there aren't the little jaggy edges that are so much fun to gnaw on.