I think you already know what you have to do.
But I'll say it anyway
People aren't just going to to come to a forum just to post (well some will, but those are the ones that just like to hear themselves talk and thier posts are usually worthless)
You need to create buzz and things for people to talk about until your forum is off the ground and your users generate thier own "buzz". Even after your forum starts to take care of itself as far as topics of discussion your still going to need to keep the content fresh.
So in short you need to have good relative content for people to show thier interest and start posting.
I myself just started a new forum as well and I post on the forums like I have a large member base (even though I dont) to give those viewing and joing something to talk about right away. So far its working and the number of registered users along with posts from others rises every week from them posting on the threads I have started.
Well thats my .02 cents
I wish you luck with your site/forum