You ask really complex questions in such a simplified way. :lol:
I think there are multiple methods. That goes from everything such affiliates, search engine optimization, signatures, and of course, forum promotion. 😀
As Rick Ace said above, there are multiple methods.
First of all you, as the Admin and Owner, need to be active on the site, creating some new topics (but not too many) so that a constant discussion can be going on. I'd also suggest trying out a contest if you have not already. The only problem here is you'd need to make sure members keep on coming back after the contest has finished.
Finally it really does come down to advertising. Remember, advertise in more than one place, and in different ways. You will only ever get a certain number of users from promoting on promotion forums, so try buying ad space on other sites.
My forum has been kinda inactive lately, it might be from the fact that I barely go on forums these days and have left the daily administration to my other staffers. I suppose I could try to help promote more when I have free time.