Every time you notice that you are biting your nails or have the hands close to your mouth slap yourself in the face. Do it hard the first couple of times trying to surprise yourself. I ended years of nail biting doing this, in fact it works so well that I even wrote about it a couple months ago (in spanish: http://deadsunrise.net/post/10-Como%20dejar%20de%20comerse%2... )

You can do other things like screaming or standing up fast or some other unusual action, it will help you to identify the habit and stop it.