Best way to promote a business forum?

Hello everyone,
I am looking for help with promoting my business forum, DiscussBiz (owned by Patrick and I), because we are currently stumbed - we purchased the site for $150, then we tripled the stats that were as followed:
Members: 31, posts: 150.

We have been submitting the forum to directories, engaging in post exchanges, and having our site link located in our signature on other forums.

What should we do? We've been adding some business-related articles, posting some ads on other forums, but the activity is so slow - some days go out without any posts at all. We've been adding new threads, but no polls yet, but that's the case - those threads are receiving replies, but the business forums aren't that activity.

So here is the question: What should we do to promote DiscussBiz?
And if you were the owner of DiscussBiz what would you change / do differently than what we're doing now?

Regards,
Carlos Andrade