Work on your SEO. Search Engine + Content = Visitors.
Social Networks, advertise.
Start small. With 20 members you don't need 7 staff members and 50 empty forums.
Hold competitions on other sites and forums regarding your forum.
Also; If your forum niche is just another "General" HF Copy, it's not going far.
Why would people join your forum when they can join 10 000 other ones with the same subjects?
linkz has this e-book which had some useful information of creating a forum.
Not sure if he's still selling it, try and ask :yus:
Visitors, well you can just invest in an ad campaign and posting showcases on various forums/blogs.
Members, is a bit more complex. You have to offer something unique or worth that time to register. If your forum niche is the standard "gaming" or "general chat" forum, it won't go anywhere. It has to have some unique factor on it.