1: Fake accounts, create a lot (~50 or so) of unanswered threads. And some answered of course.... But mostly unanswered.
2: Go to
http://www.google.com/blogsearch . Search for something related to your niche. For example, if you have a recipe forum, search for "carrot cake recipe" and have a prewritten comment like "Wow, that carrot cake looks amazing!". Post your URL in the URL field.
Post the same thing to as many blogs as you can.
That's how I started my mainstream forum and it's pretty active. When people think you actually comment to their post a lot of them will visit your URL.