It really depends what the forum is all about. The topic of the forum will point you to the direction that you have to take for the advertising campaign. If you care to tell us what the forum is like, maybe more WHT members will join this thread and share their experience.
I think the most important thing is to provide good forum support at the first stages. Don't allow spamming posts and answer all questions yourself in time till your forum is not popular. Once it is popular you will manage to take a deep breath