Find other sites that are related to yours but are lacking a forum and ask them for a link. If they ask for any form of compensation, tell them you'll be more than happy to cover their news and updates on your forum (which does generate links back to their sites) in exchange for the link. You can also tell them that by linking to your forum, you can save them the headaches of having to maintain their own. It's kinda like a free sponsorship deal that benefits both sides.
try also posting in other forums and placing a link to your forum in your signature. Then your link is visible by people who like forums
Try 3 way link exchange with more and more sites with your targeted key words. and also you should update your site regularly.