I usually just ignore it or say something like "if I thought you smelled bad I would have said something... come here and kiss me silly". Take her hand, pull her close. Be the man with a confident smile. Playfully teasing in a sweet way is what you're aiming for. Leave your negs/cockiness at the door.
I've noticed that the more insecure a girl is, the more she likes her man to be confident and balanced. At least that way she doesn't have to worry about YOU wussing out and thinking/saying stuff about her behind her back.. just be straight with her all times and she will see you as a rock.
My current girlfriend is one of those shy/insecure girls, so this is an area where I've gotten quite a lot of practice lately
EDIT: Yeah, what Locke said. LOL