Not really a big fan of advertising on forums myself. If you are involved in a community however and provide helpful feedback, then usually you'll pick up clients through that medium. It seems that every host (and their mom) posts in an OFFERS forum and you're competing bottom dollar for potential clients that will leave your business within a few weeks/months at the drop of a hat.
If you have a good working relationship with your customers, and forum members, you will almost certanly bring in new customers. Blanket advertising may as well be a flyer on my door handle at home. I'll likely rip it off and trash it before I even see what it said.