Flex,
I agree, offer something for the first "x" amount of people that sign up. I have also heard of people having success by paying people to create useful threads as members and posting them or acting as an authority on the forum to answer questions. People have to have a reason to go to a forum and typically that reason is because there is a community on a forum that helps them accomplish their goals or otherwise provides useful content. If they can't find those things, they don't want to sign up.
Maybe you can find some ideas here,
Hope that helps,
Shawn