in the beginning many start with paid posters ! you can ask for members here itself by offering at least 4 cents for each valid post and 8 cents for each valid thread ! { these are ideal payments but many pay 2 cents per post and 4 cents per thread } BTW , I will not be contributing as a paid poster
Advertising through forum signitures on other related forums can also be a good way to get traffic. Almost all of my forum's traffic comes from signitures on other webmaster forums.
Invite your friends. Email your business associates. Print flyers. Include the URL on your business cards.... But it will all depend a bit on the niche of your forum.
In the beginning you will need to get content inside the forum. No one will stay 10 seconds on a forum that doesn't have something to catch their eye or make them think "what's in it for me". Then you will need to get some back links for your forum by hiring someone or doing it yourself. After you have done that you will need to advertise, advertise, and then advertise.
In my opinion, you can't start a forum if you don't know the subject. If you know the subject then you've probably been on other forums within your niche. What are the other forums lacking? Could you do it better? Give the member a reason to join your forum. If you know the subject, you can post like mad on the forum to make it look busy. For the first 2 weeks at least, you need to be filling the forum up with posts. Even create a few users yourself and kid on they're genuine. Ask other websites in your niche (not forums) to add your like to their site, or exchange links. Add the url to all the signatures on the different forums that you use. Get as many backlinks as you can and your results will be higher in the serps.
I noticed some people purchase databases for forums. For example some existing databases with about thousands of members and thousands of posts, but I don't recommend that. Advertising is the best way.
wordofmouth is the best thing for forums...SEO is a slow process but word of mouth is essential until SEO kicks in..