this will sound a bit dorky but here goes i get quite a few hits on the site but people just don't register to post messages. i know that i can't force them to register and post(****) but if anyones got some tips for increasing trafic i would love to hear them.
Also make sure that none (at the most, a very small number) of your catagories are visable to non-members. "Forcing" people to register to view the general "thoughts" of the board are a bad idea. Only have "private forums" for mods / admins etc. Make sure that the popular catagories are available to all.
I agree with andyjenkins. The strategy I used from the very beginning (almost) was to give most of it (access and content) away. However, I reserved a few strategic areas for members only. In my example, it was the photo album. The rest of the site was accessible to all, without membership but if you wanted to see the photo album you needed to register.
This paid huge dividens for me. The day (early in the life of my site) I made this change new membership has been steady and increasing. Then I went back and created new content to allow me to take advantage of this approach. I went out and recruited a discount for our "members" from a local shop. Armed with this I was able to offer another members only incentive.
My last example, I promise. I created a new piece of content, a document. It was a collection of information that my typical user would want. As a member you can download the document for free. No membership, no document.
Get creative. You will come up with some ideas to persuade users of your site to become members.