Advertise on as many forums as you can. Possibly paid advertising as well. Do post exchanges with other users on FP and other promotion forums, this will make your forum look active and the future visitors are more likely to join your forum.
Try out all the advertising services on FP - http://forumpromotion.net/services.php
Such as banner promotion and bubble ads, this will make other people more aware about your forum and hopefully you'll get more member.
Like Tejmal said, advertise and promote your site into various forums and sites. Look for link exchanges on high traffic sites. This way, you can get more search engine visibility and possible new members on your forum.
I was also looking something like this post to get an answer, PPC needs a lot more money for advertising, forum promotion on other forums can be effective.. but i am also interested in knowing more other options..
Also, put a link in Signatures on forums - Google and co will find your forum and index it quicker, allowing visitors to find your site via Google Searches.