Not to sound narcicisstic, but poly, whether amorous, sexual, or fidelitous, is really all about the self. Sure, the word poly means many, many loves, many partners, many friends of differing flavors, but it is all about you. In my opinion, a successful poly lifestyle can only be gained when you confront those childhood fears, deal with each insecurity as it comes to the surface, and try to live as honestly and responsibly as possible. You cannot control anyone else's actions, feelings, or thoughts but your own. My wife, isarma, effectively summed up an idea of 3 simple rules to live by: 1. Have fun, 2. Be honest in your communication, and 3. Be responsible. This works for any walk of life, but certainly for the poly lifestyle, and especially to this particular point. If you can't accept yourself as you are, if you can't be ready or able to grow, if you can't take responsibility for your actions, then you'll never be able to accept the individuality in your lover(s).
Something said earlier was about control. You will never effectively be able to control your lovers, not how they feel, how they act, how they fuck, etc. Your biggest choice/challenge is to accept that the choices he or she is making are the right ones for them. To that end, if things go badly in their outside relationships, all you can do is be there for them. This is not to say that you can't have your own opinions about a situation or person, but have your feelings about it, voice those feelings if necessary, and let them decide on their own how they handle said people and situations from there. Exceptions can be in matters of health, such as if you and your current partner don't use any protection, and he or she decides to get involved with someone who has known risky sexual practices, but even then all you can do is limit the level of contact you have with your partner. I.E., you decide to start using condoms if you feel that you've been put at risk for something, as well as possibly limiting the level of emotional intimacy for some period of time. also, if your partner's emotional/mental decisions get so far out of your comfortability zone, you can always remove yourself from the situation or relationship, but bullying them into leaving another partner so that you can stay together is always asking for trouble. In my experience, human beings quite often have to experience things for themselves to be able to make a "right" decision about any given situation. Your level of personal responsibilty is under your control, and no one else's, and vice-versa.
The poly life is all about choice and freedom, as well as responsibility. Every poly-minded person has the freedom to interact, to share, to indulge, to discourse and have intercourse. No one person can satisfy your every like and desire and area of interest. This is, in my opinion, a great positive for poly. You may have cetain interests, say horror movies , with one partner, while another may fulfill that desire to cliff jump. It can be mundane, it can be literary, sexual, whatever. You may find that, if you try to put all of your eggs in one basket with one lover, then you will end up frustrated and even lonely. Expecting your lover to like everything you do, to do everything you do, to want everything you do, is just silly, and at worst, can lead to growing apart.
Jealousy is one of the unfortunate mainstays in the poly life. You are going to be confronted with the feeling, possibly often. For me, jealousy stems from something wrong in me, not the other person. Getting jealous about a feeling, a person, a situation, that my lovers are experiencing, has been brought about by a couple of things: First, it brings up a past hurt or feeling of loss in my own life/relationships. Second, it can be the childlike feeling of "Aw, man! That's so cool, I want to be involved too!" While the second seems less dangerous, they both can be destructive. This doesn't mean that jealousy can't be turned into a positive thing. You feel it, that probably means that it brought up something you weren't aware of, and so dealing with it when it happens and overcoming it is making good out of a bad situtation. Jealousy and the need to control often go hand in hand, and can lead to a number of destructive situations, like feeling threatened by a partner's lover(or even potential lover), and possibly trying to control who they see, when they see them, and so on. It is something that you once again choose/are challenged to either overcome or get swept away by, and I've seen it and experienced it first hand destroy good relationships, as well as stop good relationships from ever getting off the ground.
Here's where my failings come in to play. I have definitely grown over the years that I've been poly, and have come to terms with a number of things that I used to get very uncomfortable about. Unfortunately, those things that I haven't dealt with, or have been stuffed down, or things that I have sacrificed, have come to a head recently, and those have affected my poly life dramatically. I have a martyr complex to the degree that I give up parts of myself for the "greater good", whatever that is, and I also find that I will often put myself out there so much for others so that I don't have to deal with myself and my issues. I also have communication issues, specifically with my wife, but with others to a smaller degree, where I will hold stuff in, I will not be vulnerable, and essentially not be available. These are things that I've had issues with for most of my life. I discovered that the martyr problem may stem from dealing with my drunk of a mother for so many years, refusing to deal with her, help her in any way, to ignore, her, and that may have left some survivor's guilt? to wanting to help others all the time, even to the detriment of myself. The point I'm trying to make here is that, because I have not been taking care of myself, because I have not been dealing with my own shit, being comfortable with me, that it has bled over into my poly life. I have avoided overcoming my shortcomings to the point where I have burnt myself out, and made dents in my poly relationships that may take a long time to recover from.