Assume that you are in a poly relationship for a moment ... how would you introduce others to your situation? Like a very smart girl recently said, "Being poly is complicated".
Sadly at the time being, saying something like "Hi, i´m poly, how are you" is simply a bit too overwhelming, maybe too unorthodox, too extreme... simply not accepted by society at large. But things change in the winds of time, just like one can now mostly openly state if they have a preference for partners of the same sex, without it always being something that scare people off. Oh you' re gay? That' s fine. The acceptance of alternative lifestyles slowly grows and hopefully so does the understanding.
The first impression is still guided by cliches, by the things forced deep into your perception by society and media. Girls with tattoos on their lower back are easy, men with long black hair have to be metal fans/hippy/rockers/clicheofyourchoice, a woman that prefers black as her clothing is a goth, someone with a certain haircut is an emo and poly people sleep around.
How can one break all these opinions people grew up, things they rely on to form their perception of the world? How can you tell them they are wrong, how can you tell them little (nothing?) could be further from the truth?
I can only give you my opinion about the topic, but a lot of this is based on the context of your communication after all. I choose to approach it gently, start relating to each other... Not just you are being judged by your peers, but your peers are also being judged by everyone else. Share your opinions on the "forcing a label on others" topic, let them come to the conclusion that not every cliche or idea they may get at the first glance is true.
Maybe play with describing a poly relationship, without saying the word poly itself. Talk about your relationship where you share everything, your desires and wishes, where you let your partner know when you enjoy something in another person, how your trust is so deep you don´t need to hide your own wishes and longings. If they accept the idea at its core, without jumping at the reflex of "poly means fuck.-buddies", you at least have a start.
On that note, I was quite plagued with... when do you tell someone you' re poly? No matter if that someone is a friend, a love interest, a possible or current partner... they do have a right to know. Should one hide it, let them get to know you without the "negative" impact of knowing your poly? Or do you say it right away, scaring the other person away, maybe even more as if you would have said "I' ve got a boy/girl-friend?"... and yes, from my experience if you say "I am poly" people don´t go "Oh sweet, possibilities!", but instead think you are less trustworthy than a normal person... all while a poly relationship is based and working because of being trusting, open and honest.
Now I for once have chosen my stance on the subject, I will rather state it too early than too late. First of all, I am not ashamed of my choice, in fact i´m very proud and happy to have the strength and luck to be in a poly relationship. I also think the other person deserves to know, to be warned in a way, but also to LEARN what it means to be poly first. Just like an exciting poly relationship should be based on honesty, a new starting or adding relationship should imo, start with just as much honesty.
The right time is not the end nor the start, but do yourself a favor and have your "coming out" before anyone would be hurt by you saying it.
Additionally, you owe it to your partner. Not admitting you' re poly means that, in a way, you' re ashamed of admitting his/her importance to you.
Even if it fails and the people you tell it choose to be mono over everything else... let them be.
Don´t try to force being poly onto someone, I know those who are deeply mono enjoy ranting, spitting hate and trying to enforce their limitations onto you, but there is no need to act the same way. The freedom of choice is something I wish to grant everyone, by giving them all the information they could need to make that choice in their own, best interest.