I made links to them off my main sites. "Free" is a good word to get people to look.
Also mentioned them on other forums I was on. Make sure you have your meta tags done well, so that the forum will appear in the search engines.
One key. You have to advertise it as a forum someone 'wants' to visit. It could be the best done forum in history, yet there has to be a reason for people to be compelled to visit. IMO