First thing you should do is spread your hands out in front of you and put a direct light on them. This way you will able to take a good look at them. Look at how bad they look. Now, visualize that you have long beautifully groomed nails. Try to keep this image in your head.
You need to keep your hands and mouth busy. When you get the urge to bite, clip, or pick at your nails, stick a piece of gum in your mouth or chew on something crunchy. Find something to do with your hands. Maybe you could draw or write, just anything to keep your hands otherwise occupied.
Also, you could try artifical nails. Maybe if you have these on, it will make you less likely to mess with them.
Nail biting can also be a medical problem in some situations. Your nails contain the minerals, calcium and magnesium. If you have a deficiency in one or both of these minerals, your nails could be a source of replenishment in response to a reward system created by your brain. This may be something you want to check into.
Hope this has helped and you will soon be on your way to recovery from this habit! :)