Mulder and I get the question with a fair degree of regularity – “How do you two do it?”
Sadly, there isn’t a clear, single answer, but I’ve thought about it a lot and I have some pointers for people.
- Read read read. There are a ton of resources out there, from the free and easily found to books. The bigger point here is: While every specific circumstance is different, the broad strokes repeat themselves. Someone out there has been in a situation you can learn from before, and poly people like to share their experiences, so learn from them. Especially their mistakes.
- Think think think. What is it you want to get out of your relationship, open, that you’re not getting out of your relationship, closed? Do you trust your partner enough for this? Do you trust yourself enough for this? What form of open relationship do you want? There are lots of options, after all. How do you react when you get jealous? (And you will.) Does your partner even want nonmonogamy, or are they going with it out of desire for you? Do you realistically have the time, energy, money, and other resources to support an additional relationship? These and dozens of more are questions you have to consider.
- Talk talk talk. One of the biggest things you can do to make your relationship work is communicate, at an appropriate level of honesty, with your partner(s). And when I say the appropriate level of honesty… well. I reveal myself as an utter pragmatist. Total honesty is not, for most people, a good thing – an honest response to “Does this make my ass look big?” could be a real problem for your plans for that night. However, some questions, like “Are you in any other relationships?” or “What’s your STI status?”, do demand an instant, honest response. And a thousand shades in between. F’example, if Mulder ever asked “Have you ever broken The Rules?” I’d say no in a heartbeat. If he asked me if I could count the number of men, other than him, I’ve had sex with during our relationship, I’d ask why he wanted to know. Not because I was offended by the question or anything, but the question would be very unlike him. (And incidentally, no. Unlike some guys I’ve slept with, an an ex of mine, I don’t keep records.)
- Think, then react. The first time you see your S.O. with their other S.O. (O.S.O.! How convenient!), cuddling and kissing, or the first time they spend the night, or the first time your girlfriend makes her best dinner for her girlfriend’s mom… Lots and lots of things can make you jealous. Or angry. You can’t control your emotions, and you must remember that. What you can control, however, is your reaction. You can ask yourself why you feel jealous. What’s threatening you? Or, more likely, what are you inadvertently perceiving as a threat? Then you can try to address the situation rationally, if any change is needed. (Of course, all of this is great in theory. It’s hard as hell at first.)
- Live your priorities. I love Hal more than I could ever tell him, but Mulder still knows that no matter what crazy drug-soaked adventures I might have with Hal, I’ll always go home in one piece to be with him. And if, as we’ve discussed… and discussed… and discussed… *cough* we do ever adopt, then a lot of said drug-soaked adventures would come to an end. Or at least be /severely/ scaled back.
- Love. That’s what the whole thing is about, right? It’s not easy, but holy balls is it rewarding.
Some books/blogs/resources I recommend:
Polymorphous perversity (Y'know - taken out of context, I must seem so strange...) 
I suspect that many people think poly sounds cool because they think they’ll get “more” – more sex, more love, whatever. I doubt those people are prepared for more work!
I’ve never been in a position to seriously consider poly, so I can’t claim to understand it from a personal perspective. But I’ve learned to appreciate what it does for you, especially in recent months when you’ve been so happy. 🙂