Hold a contest? Find something that people want and give it to them, chances are as people are winning the contest they will become interactive with the community and even after it's completed they will stick around.
This is what I'm trying right now, first 20 new members with a minimum of 20 posts have a chance at a 20 dollar gift card. Just about everything goes so it wouldn't be hard to get up to 20 posts either.
Why is your site better than any other site? Why should I THE VIEW AND OR CONSUMER waste my time and go to your site.
A common marketing plan for new people is to simple list the reasons WHY your site is better. Then take this list and go out and show the world. Or what you could do is point out what is wrong with another site.
I know you have seen commercials where they say " My product is better than the next product because the next product sucks at this and this and that and we DONT.. so buy our product. "
The only method that ever worked for me was to find a lists of different promotion sites, and post on 5 different sites on different days. This way each promotion board that you go on may have different members to get different exposure to.
And you cycle with different boards every day. Once you finish your list, then you start back at the beginning and work your way through the list again.
Of course you balance this with posting on your site regularly in addition to your other stuff.
I've heard that doing youtube videos can help as well as promoting though facebook and twitter.
If your forum has a specific topic or niche, find other forums that relate to your topic/niche and put your forum in your signature there and make posts on their forums and you may get some traffic off of their sites.
I hope that this helps and good luck with your forum.