I guess, we are a little strange in that the only rules we have are what we will and won't do with others, but because of our individual comfort levels and not that we feel it will 'hurt' our relationship.
Examples: My wife is not ready for anal play. Even with me it is still in it's infancy. So our rule is 'the back door is closed' for her. My rule is that my tongue stay firmly attached to me, so one of the ladies I played with in the past will not be getting repeat business as she tried to suck my tongue from my head with her mouth. Pain + me = no play.
If my wife or I are comfortable with a playmate, then we can play... that simple. (generally most applicable at a house party or club) When we have 'play dates' we usually already know the couple or pair we are getting together with. When we are meeting a couple only for the first time, then we do so for either dinner or through mutual swing friends at a kind of mini party so we can get to know them. Only then do we decide to play or not.
There will be situations come up where one of your rules will seem... well, silly and you will want to throw it away... make that decision BEFORE play starts. As someone else said, only change the rules away from play with your spouse/partner.
The advice about the play bag is complete gold... follow it!!!
Ours contains condoms for me, lube for us both, a robe for each of us, some sexy lingerie for her and our 'play cards' (business cards for our playmates).