A little trick i learn't was to keep you forum unique from design to the forums category .
When starting a forum gather some friends to help you start off trhe forum so when you have new members joining they will always have something to talk about since this will help to keep your forum active.
Well, I think content is a modest motivator, you need to know the trick how to reach to specific Mass with some of quality content and by reaching to that point content will begin to generate itself and so on it will increase new members. Apart this forum looks also play vital role. Just my thought!