I just told myself that it was a bad habit, and that I wasn't going to do it anymore.
Went to the drugstore, bought a good pair of fingernail clippers and put them on my keychain.
Everytime I got the urge to bite my fingernails, I would first clip them so short that I couldn't bite anything off, and then find something to keep me occupied other than thinking about biting my nails.
I did backslide a few times, but I haven't bitten them in almost 20 years.
Now I am neurotic about it to the point that if I break a nail and it needs to be cut off, I will not bite it. I am always afraid that I will start biting again. If I don't have clippers with me, I will look for scissors, or go and buy a pair of clippers.
I still miss nicotine and some of the other things that I used to use, but quitting nail biting was the hardest thing that I have ever done.
Now if I could just stop eating so much!
