I bit my nails up until my senior year of high school. One thing that I did was i wrapped my fingertips in bandaids  (I always bit my nails too short and they'd bleed) so I couldn't get to my fingernails. It sounds weird but it helped me a little bit.

There were two things that stopped me from my bad habit: my wisdom teeth and to be a bit more feminine. When my top wisdom teeth came in I couldn't bite my nails (or anything) normally and it hurt to open my mouth and that jump started everything. So my second reason was vain and it probably wouldn't help you any. I was really embarassed of the way my nails looked because they were little more than raw little stumps and I always hid my hands in my pockets to hide them. 

After my wisdom teeth prevented me from biting down on my nails and I told myself "you're going to stop biting your nails because you want them to be pretty"...I just kind of stopped? (What is sort of ironic now is that I keep my nails long)

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[*]Chew gum or do something to keep your mouth or hands busy. (This seems to work well for me when I get the urge to bite my nails.)
[*]Use clear nail polish  (I was also told there is nail polish that is made for nail biters.) Someone also told me to put something spicy on my nails (like rubbing a pepper on them).
[*]Catch yourself when you're biting your nails and make the effort to put ypur hands down. Sometimes peole don't know that they are doing something because it has become "normal" to them. 
[*]Have a goal. It's easier if you have some sort of motivation to keep you going.
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I wish you the best of luck~