After being a compulsive nail biter for 42 years, my final cure was to have dental surgery. I had to wear these plastic splints over my teeth for about a month where I couldn’t chew my nails if I wanted to. Not that I recommend this method, but I haven’t had a problem since.
On the other hand, now I have to go through the hassle of trimming my nails every week or so.
I’ve been biting my nails since I was about three years old, and I’ve tried nail polish, false nails, and the bitter nail polish. Didn’t work.
However, I haven’t bitten my nails in a week. You know what did it? A tub of body butter bought on a whim – I discovered I enjoyed putting in on my nails and massaging it against and into my cuticles. My nails and cuticles got nice and moisturized which lessened my desire to bite them. My nails grew longer and to keep myself from biting, I’ve put on nail polish.
I’d been biting my nails for about 20 years. I remember in my previous attempts to kick the habit I would always catch myself biting. Now I haven’t caught myself accidentally biting even once in the past week.
Just wanted to share what really seems to be a breakthrough for me! 
I keep my nails cut most of the time for piano playing, but I have a bad habit of biting on my thumbnail.
Depending on where I’m at, I either sit on my hand, find some way to doodle or fiddle, stuff my hand in my pocket, or put something really bitter on the nail. All of them work to some degree.