the best way to get traffic to your forum is to put quality content on there. you can get quality content by
1. putting it together yourself 2. asking to license it from someone else in exchange for a link back to their site 3. there are also plenty of completely free content; check out wikipedia, creativecommons.org for some resources.
once you have good content on your site, attracting eyeballs will be easier, and you will have a good reason and something of value to offer when promoting your site.
if done properly, this can lead to a substantial revenue stream once your community is snowballing and growing quickly.