Every time you notice that you are biting your nails or have the hands close to your mouth slap yourself in the face. Do it hard the first couple of times trying to surprise yourself. I ended years of nail biting doing this, in fact it works so well that I even wrote about it a couple months ago (in spanish: http://deadsunrise.net/post/10-Como%20dejar%20de%20comerse%2... )

You can do other things like screaming or standing up fast or some other unusual action, it will help you to identify the habit and stop it.

I quit that years ago (almost).

Start by not biting one nail (left pinkie is good). Get that down (take pride in it. make it your friend :) ), then 'save' the left ring finger too. Carry on up the hand, but jump to stopping the whole left hand as soon as you're confident (become a right hand biter!).

Then move on to the right hand pinkie etc.

I made it as far as my right thumb, which I still chew when I'm thinking.

Germs and such on fingers doesn't appeal to me. Instead I just pick at them...it's just as bad. I tend to do it most when concentrating hard, and I've been doing it for as long as I can remember. Any tips? (I think I need nail clippers always available)

Ditto. I've found keeping a pair of nailclippers in all of the locations where I sit regularly (work office, home office, next to my bed, etc.) helps a lot. Nail trimmings everywhere is my next challenge.