I had to do this with Gralson when we started dating. I just told him, "You need to know that I'm polyamorous. That means that I don't subscribe to monogamy, and that if someone comes along whom I want to date, I'm going to date them. If you want to date me, it means accepting that."
As difficult as it can be to broach the subject, it's always harder the longer you wait. Wait long enough, and you open the door to feelings of betrayal and dishonesty. The sooner you bring it up, the more likely they are to trust you when you tell them you believe in honesty and open communication.
Some people find it's easier to broach the subject hypothetically... e.g. "Hey, I saw this video online yesterday about polyamory. You know, people in open relationships? What do you think about that?" If they shut right down, then you can save yourself the trouble and quietly cross them off the "potential lover" list. If they seem open to the idea but confused or uncertain, you can explain more about it. If you get to a point where it seems they might accept it, you can mention that you've always felt that way but you've never met someone who was willing to give it a shot, or something along those lines.
Gralson: my husband
Auto: my girlfriend