build a good blog/site/community -> launch forum -> profit
It's not my cup of tea but I know someone who's got a few rather successful forums and pretty much every time he launches one he starts a blog and becomes an "expert" in his niche. Once he gets a decent amount of readers and followers he'll launch a forum.
This isn't the only way of course but just sayin and you'll probably find the services you're looking for over at DP. I doubt this will work nearly as well though. I've used these services before and they are pretty crappy.
Once you have a following from a real website it shouldn't be too hard to entice your users to join a forum. Heck they might even pay for it...e.g. ubercamp, shoemoney, etc. (not a knock just sayin)