The link Byz gave is good. Both my girls went through a stage but they stopped. Chewing gum or shell sunflower seeds will keep your mouth busy. Once when I was a kid, I was fidgeting with a pen and it slipped and chipped the bottom corner of my front tooth.

Under the nails is dirty and truthfully it can make you sick - knowing this yucky fact was motivation for my girls to stop. Once they saw them grow, they felt proud of themselves. If helps to be mindful - the moment you bring your hand up, catch yourself and stop. Good luck Indie.