There is a short article here:
http://www.spyka.net/articles/tips-f...ing-your-forum
The most important thing though is to make users want to join - give them a NEED to join. A lot of people who will visit a forum will be members on similar forums so you're going to have to think up ways to make yours better then their current hangout as it were.
This will of course depend on the topic of your forum though. You may have a new feature that no other forum has, content no one else has or a different 'type' of forum.
A contest can also be a good way to give your forum a kick start