An open relationship only works if the central relationship is very, very solid, and both parties are committed to thorough communication. My wife and I decided more or less the week we got engaged how this was going to go, and got all our rules and regulations on how to sleep with other people hammered out before our wedding vows were written. We're coming up on our 19th anniversary this summer, so we must have done something right.
We had one bad moment several years ago, and it taught me something very valuable about "cheating". What hurts people isn't the sex, it's the lying and the secrets. The idea that your spouse has been leading a life that you weren't a part of is devastating to a relationship. After all this time, I feel no jealousy at all knowing that she's fucking someone else. But I know that if she lies to me about it, I'll be pissed.
Only slightly less important than complete disclosure is our veto rule. If one of us ever has a serious problem with the other's outside relationship, they have the right to demand an end to it. It's important not to use it frivolously, and it's only been activated once. We had a threesome with a woman who got weird on us, and my wife didn't want her around anymore, so that was that.
To me, open marriage is simply a matter of admitting what kind of people we are. I know myself, and I know that if a woman I'm attracted to were to throw herself at me, there's no chance in hell of me turning her away. I know that about myself, so why bother trying to pretend that it won't happen? Studies continually show that the majority of married couples eventually cheat on each other. I'm just admitting that I'm going to be one of those people, and finding a way to make sure it doesn't kill my marriage when it happens.
Interestingly, as we get older, our outside dalliances have become like second marriages. My wife has been seeing the same guy every Friday for twelve years now. I had a relationship going on for about five years, and it looks like we may be about to reconnect (again...we've been off and on a few times as her primary relationships fail).
Of course it's not for everyone. A couple who were friends of ours decided that our situation looked boss, and wanted to check it out. They did it all wrong, and they're divorced now. Of course, that wasn't the only reason; it wasn't a strong relationship, which is exactly why they shouldn't have gone there. And I actually feel guilty about having been party to it (I was the primary reason she wanted to do it, and we started getting together once they made the decision). The odd part is that I'm still good friends with the guy, who I've known since High School, and I haven't spoken to her since the divorce.
Sorry this was so long. I have a lot of history.