We have a stip poker game called "Naked" each card has an article of clothing on it, so if you have the winning hand you give one of your wining cards to each player, this specifies what comes off. After clothing is gone, favors work well
I also recently bought a book of party games, they go from mild to wild. Havent had a chance to use it yet but they are all set up for foursomes or moresomes. I think how you start out and with what depends on the people you are with, the boudries which are in place and the comfort levels already developed. And as someone else said, Sex doesnt have the be the end result everytime. Fun can be had in many different ways and there is always another night for additional fun if all has gone well.