Give people a reason to use your forum. Put some money into it and you'll start to see results.
A lot of people join namepros because its not only a great community where they can get their questions answered and sell their domain names, but they can earn currency to put towards domain registrations and the like. I've seen a fair number of failed fourms that try the "giveaway" at specific points in their infancy. They fail for a reason. Because the users are not constantly getting back what they put into it.
-Steve