Once people get to know about your forum thenits a matter of consistency and keeping spams off the door .But before that you will have to generate awareness through numerous ways .
I've had success with finding a prominent blogger who covers the same subject/material that I am writing about, and either getting their permission to post their blog on my site, or even asking them to write a short piece on my site. Getting other people involved in writing for your site no only makes it look more legitimate, but it also can help bring in people from that authors site.