Biting/ chewing on my lips. This is sort of a chicken-vs.-egg thing, but my lips have a tendency to get chapped, so I bite them, so they get chapped, so I... get the picture? The one bit of make-up/ cosmetics that I always, always, always carry with me is my chapstick. I wear it religiously.
But, yeah, I probably ought to try not to nibble so much.