I would highly advocate being honest from the very beginning. I would also recommend being both blunt, and very nonchalant about it. I have found with this (and many other things, like involvement in BDSM, other alternative lifetstyles, misunderstood religions, etc) if you are completely honest and forward, while also acting like it is perfectly normal and not a big deal at all, you get excellent responses. I've never actually had anyone take issue with any of my less-than-usual lifestyle choices, even people who have had problems with other people being in those lifestyles, or who had conflicting beliefs. I honestly believe it is because I have always been completely transparent, and have also not made it seem like there was anything wrong or different about what I was doing. That seems to have made people very receptive to just listen and try to understand, even if they might disagree. It has also led to me becoming involved with people who would never have considered polyamory before, or explaining it and having someone who was previously vehemently against it suddenly decide to give it a shot.
So I would advocate that you are upfront about it as soon as possible, the same as you would be if you had a child, or already had a partner and were looking for a second, or so on and so forth. If you are searching for people online, for sure put it early on in your profile. If you meet people in person, as soon as conversation turns to dating, I would recommend saying something like "well lets talk about what we might expect or be looking for" and bring it up then. That kind of frankness also tends to impress people and can get other major issues out of the way early on as well.
I hope that helps, and I wish you luck in your foray into polyamory!