Keep checklist and a nail file on hand and when you get the urge, substitute them for chewing and changing the looks of your fingernails. DECISION belongs to your brain not your mouth.
I used to do that too! Now that I have long nails, though, it feels weird when I have to file them down. Like Morgan said, try finding something else to do with your hands. I would also suggest wearing clear nail varnish because this makes it harder to bite your nails. You can also buy clear varnishes that are designed to help your nails grow and stay strong and healthy. Good luck :)