Help me stop biting my nails?

I'm about a 50% biter and 50% picker but the end result is little nail stubs where there should be a healthy nail. I also pick at my toes too if I'm sitting on the couch. I don't really mean to do it...I basically get my fingers when I'm reading and my toes if I'm on the couch watching TV. What can I do to stop this habit?