In the polyamory world, we don't give out toaster ovens for making new converts—that's for the queer people, which is just as well. I can't speak for others, but my record for monogamous–to-polyamorous conversions is pretty low.
Still, one tries to remain open- minded. Max and I were at the gym recently, and a nicely buff man working out near me asked, "So, uh, is that guy your boyfriend?" I replied, "Yes, he is." But that doesn't mean I'm off the market, I thought. However, I wasn't sure how to succinctly convey that without seeming too forward, so the conversation foundered. Too bad, he had nice triceps.
Who knows, he might even be interested in polyamory. But I'm guessing he wouldn't know quite how to proceed even if I'd replied, "Yes, that's my boyfriend, but we see other people as well." I've noticed some cultural gopher holes that nonpoly people often step into when hitting on us—let me give you some tips.
If I tell you I have more than one romantic relationship, don't assume, "Oh, she's a swinger!" There are some poly people who also do spontaneous casual sex, but those of us who don't are a little touchy about that. Also, you may have watched Big Love, but religion-based polygamy is a whole different thing, so disregard that.
If you're initiating a flirtation electronically and your desired one says on her MySpace page/personal ad/blog/whatever that she is poly, mention it when you message her. You don't have to say much, just something like, "Oh, I see that you're polyamorous. That's interesting, you'll have to tell me more about that sometime." Anyone who's sexually other has experienced someone he/she thought was cool flipping out and running away when the taboo desire is revealed. We like to be clear that you know the deal before we start down that emotional path.
Conversationally, deal with poly people's other partners the way you might deal with their children: Acknowledge that these are important people in your desired one's life. Ask a few polite questions about their work or hobbies or whatever. It's showing due respect to their existence. It costs you nothing to create goodwill with the incumbent lovers, and failure to do so could cost you: If Max and Monk told me someone who'd flirted with me had been anything less than courteous to them, well, that would be the end of the flirtation.
And a few remarks for poly people on the prowl. Disclose early and often. Monogamy is a huge thing in our culture, so if that's not on the menu for you, don't hedge about it, or use weasel words like "special friends." I recommend against missionary dating. That refers not to the male-on-top sexual position, but to trying to recruit someone to your sexual group in spite of his or her expressed misgivings. Obviously if the person is new but game to try, that's fine, but don't use the power of your pussy/cock/whatever to hook someone in spite of themselves. You're more apt to end up with an electrical fire than a handy household appliance.