participate in other forums with an link pointing to your forum, start conversations on your forum, encourage people to participate, what you need most is active threads
Make some good quality content first, create more high quality backlinks, add your site in signature http://proudengineers.com/top-list-of-web-2-0-sites-for-high-pr-backlinks-for-free/
for my C3 Corvette Forum , I have used Craigs list. For about a year now, I post 3 listings each week. One to a different state in the U.S.. When I have made my way through the 50 states, I start all over again. It's free, but very slow. It does however get me quality members that stay active in my forum. It won't get you rich quick, my forum doesn't make much money at all. It does however make more that it cost to keep it running. You need to be in for the long haul on forums. It can take a very long time, in my experience to get them off the ground.