I cannot afford paying for promotion, so the hints should be for "free advertising".
You can't even afford $10 for Google Adwords? Without some kind of boost at the beginning, you're going to grow VERY slowly.
I guess if I had absolutely zero dollars, my best way to promote would be SEO on the forum itself. For example, I just learned that Google doesn't normally index phpBB forums. The reason for this is the sid in the URL. There is a phpBB mod that fixes this so that Google will get normal URLs and start indexing the pages. By doing this, anything you post might start appearing in search results, and thus might bring in visitors. There may be similar things you can do to your forum that will "encourage" Google and Yahoo to index each post.
I might also come up with a succinct, excellent signature, add it to my profile on OTHER forums, and start posting on THEIR forums in addition to my own. Your own good posts on other people's forums will drive a few interested people to your site. The trick is to make your posts actually helpful.
-Tony