Best response to a girl being self-deprecating/insecure?

I'd like to know what experienced sedditors do when a girl may be complaining about her looks, either in a genuine way or when she's fishing for compliments. I'd like advice on a one-off instance AND for girls who are constantly belittling themselves.

I don't think saying "No you're not." or "Yes you are!" (in a joking way) are good responses. Any ideas? This one's a toughie.