Im not an expert at getting traffic to your forum but i can throw a few tips towards you
1. It's probobally best you make your forum open to the public meaning you should make all forums,threads and posts viewable to the public (minus your private forums).
2. Try to have as much content as you possible can , all the content you have will count as "key words" basically. It's alwats nice to have popular content as when someone searches for a specific topic they might just land a link to one of your posts or threads.
3. Try to limit frames or anything like this, bots that i know of can't handle the frames and your pages(s) will have a description of "Your Browser Does Not Support Frames" or something along the lines of this.
These are three small tips . they might not be totally accurate but i find they work out nicely
