There's a recent article on How Stuff Works about the 10% threshold. It explains that for some reason, when you have a community with a minority opinion (RPGs are fun) if 10% of that community starts to share that opinion it quickly flashes over to a majority opinion.
This rang a bell with me with making an RPG and getting people to play it. If you can pick a small enough community, say the frequenters of a game shop, and get at least 10% involved in a game it might happen that more people in that community will pick it up.
Then move up that scale with a slightly larger community (which I can't think of one, local con?) and repeat the process. It's possible the same could be true with forums etc.