Create fake users, generate interesting debate to encourage participation
Content, content content, for the search engines
Offer things for user contribution - e.g. a reward for a discussion spanning ten replies or more. You need to think about the sheep effect - if there's no posts no one will post..
You have too may categories too - better to have a few with a lot of posts in them to start with - u can always add more later..Looks like a more active forum then.