Find a niche market, with a large audience.
It helps if there aren't that many similar forums / sites on the internet.
Make sure you have something unique - and I don't just mean the theme!
Keep posting yourself, the more you post (they must be of a high quality though) the more likely your forum will get noticed.
Try getting links from similar websites that are already established (even if its only a link in your signature in another forum).
Make sure to take a very active interest in whatever theme your website has.
Keep it small at first maybe a news / updates forum, a forum related to your theme and a general chat forum. The fewer forums, the busier your forum looks.
Try engaging your viewers by starting a debate and getting them posting.
It can take several months before you even get 5 regular members.
Get indexed by google.
Of course, i'm a bit of a hypocrite as my sites are general and don't stay for more than a few weeks - as mine are generally testing new things that I find.
Hope this helps
lambada