Advertising a forum is one of the hardest things to do, I wish you luck. One thing though, if it's the site in your sig, I'd cut way back on the number of seperate forums. Add additional forums as you grow, don't try and start out that size.
I guess that advertising a forums is just like advertising any other site. You have to throw money at it to gain traffic. I actually think that gaining traffic to a forums is much more easier than getting traffic to something like a web directory and infact quite a few other sites. The reasons for this is because when you get a few signups which get quite active then you start to grow a communitie which makes the site stickier and your users then have a reason to come back plus it also increases the chance of others registering when seeing that the communitie is growing.