And be prepared to work hard for a lot of time. Forums are not "auto-pilot" sites. It requires a lot of time/devotion to make a forum succeed.
Create something unique, something (a niche) that will bring people to your community. You have to have topics that inspire people, topics that make other people link to your community. Only by creating a spirit of a community will you succeed. Otherwise it will just be another another site.
-contests -Interesting topics -Get to know your members -Be a very respected individual at another huge forum -put the link to your siggy in every spot you can -Get on the search engines -Optimize your keywords -Mass Private message of new announcments -Google adwords does great help -Buy backlinks from high pagerank sites -And this a mistake admins make a lot. When someone pms you. PM THEM BACK!! When someone posts, you post back. -Get staff. Staff are good at most times because they feel the place is a bit theirs so they are motivated to spread the word. But probably the most important thing is the design of your forum. Make sure it is welcoming to all ages, sex, and locations. I see so many people creating some amazing dark skin that has loads of great Gfx. That looks good, but then you'll find that most females and people who are not teenagers will not join it.