Replying to Joe's request in another thread, here are some ideas I would suggest exploring to make forum memberships more attractive to users. I realize these may or may not be possible with the forum software, and these are just brainstorming, not necessarily things I would want.
1- Restrict PMs to paid members.
2- Restrict non-paid members to 10 posts/day.
3- Restrict personalization options to paid members (sigs, custom avatars, etc.)
4- Allow paid members votes on forum policies, etc.
5- Work out a partnership with Nashabr/Performance etc. for special coupons for members only.
6- Develop a BF.net store (e.g. use cafepress.com) and make things available to members at a special price.
There was something else I came up with on the way home... but its gone now.