I guess you should go for some ad space banners in well known forums like DP. Also Facebook is also not a bad idea. Try creating a fan page on the forums name and start posting stuff what is there on your forum. You can even join some SEO related groups and fan pages and ask for their reviews for your forum to catch some eyeballs
Get registered to the related Blog, Forums to your forum. Then post content, comment etc. FB is a great medium .
Provide quality content on your forum each day. Submit your quality posts to the social bookmarking sites.Keep posting daily.. as much as you can.. more you post, you will get more traffic from visitors.
Best forum building is through linkbaiting, get some discussion going, create controversies at your forum threads and you will see spike in traffic or just PRAY
Try to advertise all around the internet of your forum link and hire forum poster with good offer by which they came to your forum.They will post good quality of threads and postings.Also add buy,sell and trade in your forum by which they can sell their forum signature to another forum poster.