Here's a good trick to get people to your forum
It's a trick a buddy of mine taught me..
Let's say you create an a forum for cars, and you want to bring on as many car enthusiasts on there as possible. Here's how you do it:
You find Twitter accounts that car enthusiasts are likely to follow. They could be a competing forum, a fan site, a club, etc. You simply go their Twitter pages, and look at the people who are following them, because the people who follow them are likely to be in your forum too. Then you follow them, because each time you follow someone they get an email or notification from you, which prompts them to check you out.
I've notice when I do the upper limit of 1000 followers per day, about 300 people follow me back, and about 100 people go to my website. The people who did not follow me back, I unfollow to maintain a 1:1 follower:following ratio, which is a Twitter requirement once your account gets large.
The biggest challenge with this tactic is filtering the followers to avoid inactives, bots, etc, and to unfollow the right people. Luckily, there's a tool that does this for you over at http://Flutter-app.com that you can try for free.
I'd recommend following 50-100 people per day to start, otherwise Twitter might see your behavior as aggressive and suspend you.