Chances are if you were a child nail biter, your parents tried painting your nails with a bitter polish at some point to dissuade you from putting your fingers in your mouth. Make sure if you try this technique that you buy one of the polishes that's specifically made for nail biting cessation. This will ensure that you aren't ingesting the nasty chemicals that make up clear nail polish. The idea here is pretty rudimentary -- the gross, bitter taste on your nails should make you want to quit putting your fingers in your mouth. The key here though is using the stuff. If you're a nail biter, you may be reticent to use a bitter polish because you're so used to biting your nails. Regular use over the course of a month should help you kick the habit, though you may need to extend beyond that.