I second the yahoo answers I get traffic and good backlinks.
You can post in forums, post free ads in free classifieds, you can post in the forums and blogs section on my site and get steady traffic. Make sure to post a pic with your post so your ad will also show randomly on the home page for life. My site is in my sig.
First of all, what does your forum offer? If it's a computer forum, for example, check out computer sites and start acquainting yourself with the community. Then tell them that you have a forum and are looking for readers. If you have a forum about TV shows, acquaint yourself with the TV show community and push your forum on them in due time.
You can't just have an idea but not execute some way of marketing it. Of course, a lot of forums don't succeed because their owners are too busy posting on the forum (by themselves) but not doing the necessary outreach to promote the awareness of the forum itself. In the beginning, you're going to need to do a lot of both -- community building both in the forum and outside the forum.