To use a lipstick to paint each other's lips, nipples and pussy lips/clit, and then write the name of a sexual act on each other's back. Then you take turns guessing what the other wrote. Whoever guesses right first, the other has to smear off all the lipstick with her tongue/vibrator and then perform that act on you.
To make it more exciting, while the other is putting the lipstick on you, you have to tell her how it feels - if you use the same adjective more than once (eg, 'nice', 'wierd', 'silky', 'cool') she gets to spank your ass, if she thinks you're taking to long to tell her, she gets to spank your ass.
PS: If you could make a few lip prints on paper and post them in this thread, that'd be most appreciated, I'm trying to start a collection there
http://www.getdare.com/bbs/showthread.php?t=54715