Breaking a toddler's bad habits?

My son is two.

He doesn’t suck his thumb, or hit his friends, or use a pacifier…

He pinches his nipple. Yes, the one on his chest. Just one of them, but he does it constantly. He walks around with his hand down his shirt, just going to town on himself.

When he was little, it was adorable, and I was just happy he wasn’t a thumb-sucker or nose-picker. Now, that one nipple is starting to look funny, and it would be great if we could break that habit.

Unfortunately, unlike thumb-sucking, there isn’t a whole lot of information on neat ways to keep your kid from being a perpetual titty-twister.

Help?