Once you become aware of your nail biting habit, you could try directing that energy into a different action. For some people, that means keeping their hands busy so they don't have a chance to unconsciously put them up to their mouths. A stress ball -- basically a little rubber ball that you can squeeze in the palm of your hand -- might help. Whenever you find yourself starting to nibble a nail, take out the stress ball instead. It's not quite the same, but if you stick with it, the urge to bite your nails instead may disappear. You could also get some Silly Putty and knead it with your hands.
If you'd rather do something more productive with your hands, consider taking up a hobby that involves constant handwork like drawing, painting, sewing, knitting or crocheting. When you get really comfortable with one of the latter two, you can do them while watching TV, waiting in line or riding on a bus -- all times when you may be spacing out and unconsciously start nail biting.
Rather than looking to your hands, it might be time to think about what's going on in your head when you bite your nails. Let's find out why the reasons behind nail biting are even more important than the actual act.