Join a few top site lists for forums, at the same time do some advertising for it, for example buy links and banners.
Have it based on something that doesn't already have a larger forum in place. Small niche forums do well but if you are targeting things like making money online or webmaster stuff then you will have to grow slowly OR have a nice ad budget. Going to other forums in hopes of snagging members isn't the best route, myspace is a great spot and so could any social site for that matter IF the people are interested without spamming the poop out of the place. Simple sig links and articles work wonders these days. Also advertising based on your specific niche does well "Want to hear what others who are into NICHE think?" go to yoursite.cooom