gloves, seriously, make them wear gloves inside until they stop. It is a habit and usually sub-conscious so they do it without thinking.
I used to bite my nails, my Mum tells me I did it from being at least 4 years old, I stopped when I was 15 but still bite the skin around my nails.
I went to see a hypnotherapist but he said it was so ingrained that they may not be able to cure me.
I wish my Mum had tried harder, I hate the habit and even when I make a conscious effort to stop I find my fingers in my mouth and I don't even remember putting them there.
I stopped biting my nails by wearing cotton gloves. I must try to do that for my skin.