A few hints from my experiences:
1- Start small and try to fill a SPECIFIC niche.
My 1st forum was about slots – and had some die-hard real slots players on there (about 120 of em).. 2nd forum was Poker. The best thing I ever did was then to funnel both to forums to forum 3 (a General gambling forum) and ditch forums 1 and 2. Some of my other gambling websites have a “gambling forum” link – which also directs /funnels more traffic to my forum. Result.
2- Content
Yes content is still king. I recently wrote an article (which quadrupled my main site’s daily traffic) and added a “discuss this in our forums” link.. Visitors and signups increased significantly..
3- Good deals/promos
Get your punters some good exclusive deals when you can, or get sponsorship from some companies in your niche for competitions (in exchange for advertising / signups).. Small events can build / foster community spirit – which at the end of the day is what keeps a forum growing, and alive (and it costs nothing).
Just a few ideas off the top of my head..
oh and don’t forget, Don’t click this link!