If you want to promote very quickly then use google adword services, it will give you results within a 24 hours. to promote website with organic SEO techniques you need to all SEO activities like article, blog comments and forumpost.
A forum will not become popular, if there are no interesting posts in it to comment. To make it popular, I think you should go hire paid forum posters who can create interesting threads in your forum.
Just go with Organic traffic, do some seo, rank your site for long tail keywords then go for generic keywords. gud luck!
Post about your forums in other forums. Allow users to have signature immediately after registration until your forums gets more popular.
I prefer article marketing for a new forum - Just write a article and post to different article directory and you will get permanent link / traffic to your forum
Do some major contest with cash or GC as your prices. This contest should involve dedication and loyalty to posting in your forum. An affiliate type of membership would also set a web like registration to your website. Hope that will give you some hints.
1] Invest some money on CPC. Bcause you want to earn, so you have to invest. 2] Do some SEO for regular basis. 3] Allow Signature links so that link builders get attracted to your forum for link building. These 3 are enough. Don't go for cheap seo styles.