Welcome to WebmasterWorld, ssnet. You might start reading in the Community Building Library [webmasterworld.com] - there are some great archived threads about growing a forum, getting visitors, getting lurkers to post, etc.
If your forum covers a niche that no-one else or few is covering, then you will slowly get members by work of mouth. If you are competing in a busy market then it may be worth having a few boards that are quite specialist.
Make sure you visit the forums every day and reply to new posts, make new members welcome, and start new threads to get people discussing.