Gentlemen... I struggled with the exact same problem for over twenty years, and have just recently been able to stop within the past two years. I, like you, managed to kick most of my little quirks but this one remained to be the most difficult and most grotesque. I just remember hitting it off with this gorgeous chick, and noticing her stare at my finger tips. I used to bite all of my nails into the quick, so they where half of their normal size... often causing pain and bleeding.
Here's the solution.... One day, while in the sauna after working my ass off in the gym, an older wise guy was talking to another guy about how to stop smoking. Here's what he said....
(IMPORTANT: Take you're time and read this. OnE SenTEnCe aT a timE. Invision something that you want to get rid of in your life. Concentrate on it.)
Imagine that you are at the beach.
It's a nice clear day.
Everything is peaceful, not a cloud in the sky.
You want to take a walk and let the waves rush under your feet....
You can't.... your in a hospital bed,... surrounded by your family members and everbody you ever loved (dead or alive).
They're all crying because you're dying in front of them, and...
As these people are crying hysterically around you, you just keep lighting up more cigarettes, blowing smoke in the faces of the people who love you the most.
You fill the area with smoke so you can't see your family's faces.
You fill the sky with smoke so you can't make out the sun...
This is what you will do and think about everytime you go to light up a cigarette.
His words where powerful, and every body in the sauna said, "Damn! Man you just rearranged my whole perspective on things." :O
What he had actually done was employ basic hypnosis at the perfect time. We were all loose, relaxed and open minded the way you get after working out. The things he said were arranged so that they go directly to the back of your mind, so that you can not filter their effect. Just before he spoke with the guy about smoking, he told everyone to invision something that they needed to get rid of in there life, and concentrate on that image. Then as he spoke, we applied the same thoughts to whatever we invisioned and it had a very instant and powerful effect. I also decided to pay special attention to the triggers of the urge to bite my nails.
Here's what I found... I didn't bite my nails when I was busy. It was only during the down time (which I guarantee is what you do as well). I found that while I was reading, driving, watching television, having just finished a meal, or (disgusting but true) sitting on the John I alway had a tendency to put my fingers in my mouth to bite my nails. I employed this technique everytime I would find myself in these situations and slowly deprogrammed myself, one situation (one instance) at a time. The pesky part about this habit is that you aren't aware of how many times, and in what situations you tend to engage in nail biting (I can tell by what you and others have said above).
Do this.... It works!
Take the pressure off yourself and cycle this through you mind one instance at a time so you can finally gain control of this situation. As my nails grew, I made sure to massage cuticle oil, or hand lotion into the cuticles of each finger at least twice a day. I also gently pushed the cuticles back, and made sure lotion or oil got under the tips of my nails. These two actions provided me with relaxing stimulus I got by biting my nails. My shit grew back beautifully, and the chick I meantioned earlier was noticeably turned on when I visited her months later. Futhermore, I'm a pretty strong minded guy and don't go around recommending things unless they are effective. I'm a chemist who got into great schools like Havard, Univ. of Pennsylvania and Univ. of Chicago. If that shit could work on my stubborn mind it could work for anyone.
Good luck.