What ever website you have for promotion only SEO will help.You need to follow the techniques of SEO.There are two techniques used for this.On Page Optimization and Off Page Optimization.For forum, Off Page optimization will help you better.You go through the methods like blog posting, article posting, classified posting, forum posting, directory submission, social book mark posting and press release submission.Its a time taking process.But it will definitely help you.
Don't start too many threads in the first place, yet, you should add regular topics to ensure consistency. Set up accounts on other forums and describe your forum idea there; people will follow. It takes time for sure, but there's always place for a new forum, whichever the topic/niche may be.
Some of the people posting before me have nailed it: networking!
A forum is basically an online community. People aren't going to want to be a part of it right away. First you need to establish your forum with regular thread and content updates so people can see that this isn't going to be a fad. People will feel like they can invest their time and effort into this community without it going away a month later if they see that. Also, like any new community, it will take time to grow. Just keep at it and before you know it there will be a lot of new users.
A key thing to ask yourself is, are you doing this because you're passionate about this, or because you want to make money? If you're passionate about this than it will be easier. If you're looking for a money maker than you, my friend, are in for a disappointment.