Until I was 16, I would constantly pick at my nails. I'd just pick them away as soon as they got long enough to do so. I'd pick at the cuticles too, which was gross and kind of painful. For my year 12 formal I got acrylic nails (much stronger and nicer than plain fakes!), which was cool, but the cool part was that after I got them removed, the urge to pick at my nails just disappeared.

Since then I've grown my nails. They're pretty strong, which is neat, and currently painted silver.
I sometimes pick at the cuticles still, though. I think it's a stress thing.