How to build a succesfull forum?
Offer reward or money for your user post. For example you can offer them $10 after they reach 100 post.
Start controversial thread that will attract people to talk.
Quality post is essential. You can do it by your self or hire other people to do the work.
Do affiliate program on getting member to the forum.
Advertise you forum on Adwords
Use the forum url in your signature. Do a lot of post in related forum.
Promote your forum on facebook, google groups, yahoo groups and other community websites.
Talk to bloggers and ask them to post about your forum. In exchange you can offer them back link to their blog or pay them for the post, maybe $2.5. You need to find blogs with enough traffic.