I tried all sorts of things over the years to quit nail biting and nothing worked for more than a few weeks until I discovered hypnosis. I was actually in a hypnosis class and the instructor needed a volunteer to demonstrate on. After a 25 minute guided visualization I never bit my nails again. Seriously.
The thing is, when you have been biting your nails for a long time it is a subconscious habit. This means that you often start biting your nails without realizing that you are doing it. With enough will power and awareness you can stop once you are aware you are doing it, but to some extent the damage is already done. Probably every one of us has had that experience of finding your fingers moving toward your mouth all on their own.
Hypnosis solves this problem by bringing the habit out of your subconscious, making it conscious, and then removing the underlying urge to bite all together.
Here’s a great free report all about ending nail biting, including 20 methods for quitting: http://www.stopbitingyournailsnow.com