Step1
First, identify the reason you're biting - are you bored, are you hungry, are you nervous, did you just stop smoking, are you mindlessly biting while you watch TV? Nail biting triggers take on many forms and if you can identify your triggers, you'll find it easier to eliminate them.
Most commonly people are bored, nervous, or mindless nail biters. All of which are problems that are impossible to avoid - there will always be boredom, nerve wracking situations, and downtime ... you have to stop the habit while facing the source.
Step2
If you're nail biting is severe, first coat what's left of your nail in a nasty tasting polish. You can find these at most pharmacies. Then wear lightweight, cotton gloves when you're at home. If you bite when your bored - try to keep busy but make sure you're wearing the gloves to do nearly EVERYTHING... You'll be used to them after an hour or two and when you reach your hand to your mouth - you'll get a mouthful of cotton instead of nail. You can also try displacing your chewing for a while. Chew some sugar free gum or even a child's teething ring for a few days. You don't want either of these to become another habit, but displacement for a few days can help. Keep the foul tasting polish on your nails all the time - since you likely won't wear the gloves in public, you will want some backup protection.
Step3
If you're biting is not as severe, you can still use the polish, but you might try some other tips. For women, getting acrylic nails, or even those glue ons in the cosmetics department at Wal-Mart, makes a big difference. The "fake" nail will protect your little nub underneath and allow it grow. The fake nail will be much more durable and for many people (myself included) you're less likely to chew them to bits if you pay hard earned money to have them look nice.
For men, it's a little more tricky. Men can use the clear, nasty tasting polish - nobody will notice, they CAN get manicures - though most will not. Most men are more likely to beat the habit if they try the tips for severe biters.
Step4
You can try taking pictures of your nubby nails and posting them up where you will see them often. You wil be thinking about constantly it so you won't be "mindless." You can also try keeping a little stress ball nearby at all times. When you feel the urge to bite, grab the ball instead.
Step5
And, for some people, nothing seems to work - these people can try hypnotherapy. People use hypnotherapy for everything from smoking to overeating - nail biting is just as big a problem. This works great for some, but you have to have an open mind to try it.