The best way to do it is tease them about things they couldn't possibly be insecure about. It's safe, because you can't hurt their feelings, and if the joke is extreme enough, she couldn't possibly be offended.
A good type of joke I use are fetish jokes. For ex, at a party last night.
Some girl sprinkles water on me
Me: You're lucky I have so much composure, or I'll have to throw this coke on you
Girl: ZOMG i would kill j00 if you did. It would be better if it was jello, or something that wouldn't stick.
Me: Do you have some kind of "food-on-your clothes" fetish? Like, do you go to KFC and ask them to throw chicken on your shirt?
Got a pretty good laugh out of her, and I used the opportunity to kino, c&f some more, etc. Something to think about.