well to get people to visit a forum, you need to post constantly and have a good amount of visitors to your site. i'd say if you don't get 5000 visitors a day don't bother with a forum. the only exception that i can think of is if you have niche type site where users can't already find and established forum for your market.

the best way to get visitors is to provide quality content. this will get you listed in search engines and will give people a reason to visit the site, and quite possibly even come back.

the only other thing i can think of for forums is signing up for a forum post swap, but ive never done that and don't know how well they work.