As familiar as this sounds, most likely forums like this, sitepoint, seotree, etc, and on blogs related to your topic (community promotion blogs, forum marketing blogs, forum mod blogs, etc)
^ i agree. forum marketing is your best bet. you may also want to create a squidoo lens, claim your blog in technorati etc
thor22 is onto it ! a "Successful Forum" must have members and the old " whats in it for them " I have taken it upon myself the last 2 days to search for high traffic forums , this is one of the best ! When you search you will see heaps of forums but 99% are dead in the water - they cannot maintain themselves and without post after post every few minutes why will anyone come back! Its this High traffic that gets you noticed is the SE which is the point my pennies worth a riddle for you! " I am hotforgdi which is the best of gdisecrets giving all incomelife ! "
You can either continuously post links to your forum discussions on webmaster sites such as DP or SitePoint. There is another forum masters forum called The Admin Zone which you can market your forum there as well. What I think will really build up your forum is that you must start to have a lot of info and contents within the discussions on your forum so that you will be able to get search engine traffic.