Don't Date Outside Your Species!
Recently, I was reading through a conversation thread about a person who identified as poly, and often dated people who had previously considered themselves monogamous. The experience they kept having was that their dating partners would say that the whole poly thing was okay upfront, then, several months down the road, and a lot more emotional investment, try to make the relationship monogamous, or break up with them, saying they just couldn't do the poly thing.
My first thought was to date people who are already demonstrably poly, but I realize that, in many places, the predominance of poly people isn't particularly strong, so one tends to make due with the dating choices that are available.
Here's the deal though: If someone is monogamous, they aren't going to turn poly for you long-term.
Choosing to date someone that is poly, because you like them, when one is monogamous, and would really prefer to have the poly partner to yourself, is like going on a diet because someone you love asked you to. Maybe it's healthy for you. Maybe it's something you think is a good idea. It just isn't what you really want in the deepest core of your being.
For a while, you can make yourself do it, cheating here and there with fantasies that the person you love is going to fall so deeply for you that they'll realize you are all they want in life, while still making the appropriate statements of openness and support for their poly nature, and how on board you are with the whole thing.
For a while, you can go through the motions, pushing yourself with the thoughts of how much your partner is going to appreciate what you are doing for them, how much this will mean to them, how much love you will earn. Maybe you are even realizing some personal benefits and growth in the situation! Eventually, that well will run dry though, and you're left realizing that this isn't what you really want, and you've been doing it to try and please someone else.
Of course, the same issue happens in the inverse too. Thinking that someone who identifies as monogamous is going to make the leap to poly for you is a dangerous and cruel self-deception. There is no amount of attention or energy you can give someone who is monogamous (and wants you to be monogamous with them) that will create enough safety, security, intimacy, and love to get them to be fine over the long-haul with the part where you have other loving relationships.
Mono-poly relationships are workable, just not with any real level of "doing it for the other person" present. If both parties can't honestly say that this is what they want, for themselves, because it is what is best for them, move on before hurting each other deeply.