Plan and support. Plan ahead & enlist help from friends, family, your doctor, etc. Think about the situations when you are more likely to bite your nails and the emotion behind them. If you bite your nails when you are nervous, you need to find another way to sooth anxiety. If you bite when you are bored, find something else to do with your hands. Consider chewing gum, mints, nail polish, fake nails as deterents.
Make them taste bad. I trust that you're talking about biting your fingernails. Some folks find it helpful to coat their nails with something that is bad tasting. Tabasco or some other hot sauce is a good choice but it really depends on your particular taste. You might also work on controlling your anxiety through other means than nail biting.
Treat the cause. Is it because of stress or anxiety it became a habit. Perhaps you need to find the cause and deal with the problems.