Here are just a couple of quick suggestions to get you out of the habbit (although they might not work) :
1) whenever you have the urge to bite your nails… pop a sweet or chewing gum into your mouth.
2) buy artificial nails
3)polish then in a dark colour
4)put on gloves whenever you feel like nail biting.
5)keep your hands underneath your legs
6)get a manicure
7)bandage or plaster your fingers
8)find a hobby for your hands so they’ll always be busy.