Offer rewards for the users, like the 100th post count for a user gets $15 etc. Or you can advertise on sites or offer some thing special in your forum to download, then people will sign up Lots of different techniques..
Get content on there at the very least, I know I would never join a site that seemed absolutely dead and had nothing on it at all.
Add the forum to your signature links and post here as well as on your forums. If you have something interesting to say, people will want to see what your own forum is all about. - Mike
Quality content and a line of posters (friends) that'll help you out with your small-time community until it can grow by itself.
you may also add a little section to your website, something useful to your visitor like free stuffs, tutorials, Chat room, or any thing that will give something of value to your prospective visitors.