If you're thinking of starting a new forum there are two great ways to motivate people join your forum and post on it. One way is to offer a free download in your "welcome" email that is related to your forum. Another great way is to include a "free downloads" area which is exclusive to certain members. If your downloads are valuable a lot of people will probably be motivated to post until they reach the level to where they can access that area. Unfortunately, I couldn't use the downloads area on my forum because SMF doesn't show boards that you can't access. Vbulletin I know for a fact does have that feature.