SEO, unique content, you could also think about using Yahoo! Answers to promote your forum, also Twitter, you could look into other socail networking websites, such as Facebook, but that's totally up to you, forums are a pain to get started, but once they are running you usually have a good amount of recurring traffic and also a lot of daily pageviews. If you are using vBulletin, you should look into vBSEO, which is a paid plugin for vBulletin which is designed to improve your SEO, changing URLs (For better SEO) and what not.
SEO is great and you can ask your friends and a couple of people to join. Remember no one will join a forum that has no members :/
Offer members a reason to join. Monthly prizes, points system etc... Depends what the forum is about really.
To add on to that, nobody will join a forum that has no active topics / posts. Remember to attempt to reply to every topic made and try to reply to each reply made.
Besides all of the usual stuff like optimizing your site, link building, keyword placement, articles, making lots of post, there are some other important considerations. After you have the site built, links in place, some page rank built up - after all of that, you have to understand your targeted audience. This is who, what, when and where. Who - age, sex, language, religion, interest, hobbies,,,,,, What - is your group looking for. When - is your site a "hot topic" of the day? Or is it a long term project? Where - is your audience at on the internet. Are they on places like myspace, facebook, youtube, forums, news sites, political action blogs,,,,, Once you have that information, then you can start a targeted advertising project to fit your needs.
Try to populate your forum with high quality contents that people in that niche want to read about. You might want to hire several people from the content creation section to do paid forum posting for your first few hundred posts. Try high quality backlinks from relevant blogs and article directories too because that will help to increase your search engine ranking.
First at all, you gotta pay more attention to input more and more quality post as you can. Then, only promote it through PPC and other forum.
try making forum and blog comments in your niche will help. I have a smf forum, which is free and ranks on the first page of google