By far, the best way to promote one's community is through a word of mouth system, or allowing you members to promote you. You can give them tools to do this, like a referral system or other benefits to spreading the word.
That being said, I can't help but to point out ways on how NOT to promote one's community. We all know the obvious, don't spam, because your and your community will be labeled appropriately and it will be a short time before you give up your ghost town forum.
What I see a lot of people do is spend more time on SEO than actual community development strategies. I don't mind, and can see value in SEO, but when a website owner thinks it more important that their URLs look pretty, then there is a problem here. While I understand SEO is not just URLs, many of these board owners do not. The best kind of SEO anyone can have is good content. Getting on Google is the first step to promoting your community, but only content will bring people there.