Good advice from the others. I would just reinforce that Content (as always) is key. Don't invite the masses until you've got a decent amount of interesting discussion going on. As people start showing up one at a time and see the party is empty, they'll move on.
Plenty of great forums have built slowly and I expect you'll do the same, but start with something to keep their attention. It's hard enough to get people to your site, make sure you can keep them.
One other piece of advice, Forums are great for search engine traffic. Make sure everything is optimized to take advantage of it.
Oh, and keep out the jerks!