As for tips, we would suggest setting up some guidelines in advance, but try very hard to not let it become an evening controlled by 1500000 rules.
Some situations and feelings you wont really be able to plan for until they happen, and some things you can hash out pretty well ahead of time.
As for the actual potential couple, you wont really know until you meet them and it progresses to the bedroom. You will have an idea of some interest from online profiles and emails, but in person is about the only way you will truly know if it's going to happen.
Overall just enjoy the evening. Have fun, be yourselves, and do whatever you are comfortable with doing. If you have done a decent amount of discussion and soul searching, maybe even be a bit daring.
One of the best pieces of advice we have gotten was from a couple we met at our first party. They said "think of sex as a recreational activity. Love is what you do with your spouse, sex is just for fun" That disassociation between love and sex has really helped us along, because not only does it allow us to consider having sex with others without messing with our love lives, but it also puts it at a much lower place on the "importance" scale in relation to our lives.
Say we do something sexual one night that we ended up not enjoying or have no desire to do again. With the view of sex being a recreational activity, it's on par with say playing a sporting game with someone. No big deal. Live and learn.
If sex was still tied to some deeper love connection, it would be a much bigger deal, and would invoke way more feelings.