Maybe spend some money on advertising it online. Most of the time, the first set of members are yourselves and your moderators
They have to be actively posting to get stuff going.
A contest is a great idea Sometimes people join because of the contest as well.
it must be related though. What Alex proposed sounds good Hopefully an admin sees this thread...
if you and your friends get together to post on the forum regularly and give it a personality people will come back. people are less likely to come back to a forum if they're posts are not answered quickly.
I think the prizes for post count is a good idea, whatever the prizes will be. For example you can make having an avatar a prize. I noticed that we can't have an avatar here so there you go. Or offer free web hosting accounts to webmasters that make so many posts on the forums a month? ...I don't know? ..just trying to help.