1. Choose a niche for your Forum, in our experience forums with too broad of a topic don’t take off as well as targeted a topic for a specific group of people.

2. Choose good forum software, that has a whole community of people helping with update, like Simple Machines (SMF).

3. Choose a good affordable yet professional Forum Hosting company, naturally we recommend OLI and yes we host SMF

4. Drive traffic to the forum, here’s some good Forum Promotion tips

5. Setup ads on the Forum that aren’t too intrusive to your forum members, like Google Adsense. A nice tool for SMF is the Ad Management mod that allows you to add ads to certain boards, within posts, etc.

of course results vary but there are some tips to get you started