Advertising, new features, active staff, resoruces. Depends on what your forum is, graphics, tech, music etc
Also make sure its visually appealing, not plastered with ads, easy to navigate/use and looks active. If someone comes to a website where half the forums have no posts in a year they're just going to leave again.
The best thing that I could think of is a referral program, it kicks and gets you thousands of members!
Yup, go through other forums (lots of them) and get refferals on each of them. That will get people to your site. Most importantly though, is how to make them register and to keep them coming back. So like some allready said, make it visually attractive and make sure that you have a lot of resources to share. Have something to offer, and people will come (and stay)
What actually matters is the content and the quality the website has. That is the only thing that will make a website worth visiting everyday and make a proper management so that people learn something from others and then be able to do something better
