Sure thing, Sage.
I'm the poly in a successful poly-mono relationship. I have personal association with other poly-mono relationships. Perhaps I was not applying a scientific level of data analysis to the global poly-mono pool, but I was aiming for reassurance that there are workable situations.
As for how it works, I believe the major factors are the people and the effort. Luckily, I have had many disastrous relationships to learn from, and I have always been analytical about things, so I likely have a higher than average (for normal folk but maybe just normal for poly folk) awareness about relationships.
The people aspect is pretty clear. Some people are hardwired. Some are hardwired straight, some gay. Some are hardwired poly, some mono. Guys out there should give up trying to "cure" lesbians, and ladies should not worry about (and are generally smart enough not to in the first place) converting gay men. That doesn't work, yes? Some folks are in the middle and may be predominantly one way or the other but can be swayed. The same goes for poly and mono nature.
The issue there is two hardwired mismatches. A mono wanting to fix a poly is trouble. A poly wanting to free a poly is trouble. Short term stuff might work, but friction causes pain in the long term. I'm not going to worry about trying to free a mono just like I'm not going to tell a lesbian or a homosexual man that they should try some variety.
I also am not going to put myself in a position (again) where I'm with a mono who is going to want to fix me. Learning to recognize the difference and choose an applicable partner is definitely a skill, and sometimes we learn it only after we're in love.
My mono is awesome. I spent years dealing with depression and self-destruction, but even when I want to do bad things to me I'm positive when it comes to my mono. Having a positive attitude is important. Yep, she's an idiot sometimes. Sometimes she forgets to use her "out loud" voice when telling me important stuff. But overall we're good for each other. She is also not a hardwired mono. Let's call her a softwired mono, or maybe you'd say "mono-friendly." She's not likely to ever be poly herself in the full sense, but she understands and accepts the way I am. Yes, this is rare; it adds to her awesome. So I avoided the mistake of being with a hardwired mono (even though I am mono-friendly).
The next part is effort. Talk talk talk. Oh my god, poly folks talk so much I want to just strangle them sometimes. But communication is rule #1 (right? Maybe #2. I'm sure someone will speak up if it's not #1.). We talk about everything that any normal couple would or should talk about. We also talk about what's going on with me and others. I don't make the mistake of oversharing though, and she doesn't dig into details. Her personality (see part 1: people) is such that details are not needed. I've seen many folk get obsessed about the details to negative effect (does it matter who's what went where if you're happy with you what where as is? Whose is bigger or is tighter doesn't matter if everyone is happy. Any difference from my mono just makes me appreciate my mono's uniqueness.). We try our best to be proactive in our talking. No waiting for later, and we understand that we always share based on love and respect.
Beyond talk, you must have action. I make a point to translate any NRE or potential NRE that I feel for someone into energy that my mono receives. Thus, any relationship or potential that comes up causes her a direct benefit. Thinking about how delicious someone else adds to my own hunger for my mono. Beside the obvious benefit, this reassures her that she is and always will be a part of my life. Frankly, the influx of NRE reminds me that my first (think primary in a nonhierarchical way for ya'll pedantic folks) relationship also deserves wooing and fun. One of our simple rules is that when either of us comes home, the coming home person is responsible for seeking out the other and giving them a kiss. It's a simple thing, but it constantly reminds us to connect. Even when I give energy somewhere else, I always try to remind my mono how important and attractive she is. Too many times I've seen polys let NRE blind them to the lovely they have right there already. The NRE may get more E, but that no one gets left out in the cold.
Another success factor is selection. Along with NRE-blindness, I've seen polys make partner choices based on personal preference alone. When you're in a poly situation, you don't get to think only of yourself (IMHO). So when I'm looking at a potential partner (yeah for mono who gets that bonus energy!), part of what I'm evaluating is how that person would integrate with the existing situation. This doesn't mean moving in or group time necessarily. It's a recognition that we all react to personalities differently. Will this new person's personality affect me in a way that will negatively impact others? Is this situation likely to be stable or sane enough for all? And most importantly: is this someone that my mono (who knows me well) would be reasonably (maybe not perfectly but with some insight) able to understand why we're attracted? If the person doesn't get along, move on. If the situation is likely to be full of emo and crazy, move on. If my mono would look and her and think, "Wtf, dude!?" move on. When I make good choices that take me and my mono in consideration, then we're way less likely to raise the stress level greatly, and she's reminded that even her poly's mono is important.
That's how we work in a nutshell. I actually found out just this week that she had been to this site over a year ago to read and lurk. Last night I was telling her about a cute freckled woman that showed up on the radar (bonus energy) and asked why she was looking at poly sites. Her response, "It's how you are; I wanted to learn more." In retrospect, I felt like an idiot asking why she'd be doing research. Duh. She's very thorough about things that are important to her.
For the record, I've been in poly relationships (interspersed with mono ones) for over 16 years now. I finally gave up on trying to fit the mono mould and began self-identifying as poly (without intention of ever trying to be mono again) about five years ago. My mono and I have been together for about four and a half years, married one and a half with a six-month old son and two cats. I also like lasagna, freckles, and intelligence.