A variation on the theme:
Car forums often have something called "sponsor members." A sponsor member is somebody who donates to the site. It can be a donation of any size, but is typically $25 for sponsor status.
This membership could give you a myriad of benefits: Access to a special sponsor members forum, a custom signature, more PMs, a quote or nickname under your avatar, a free sticker/mouspad/mug/t-shirt/whatever (maybe based on how much you donate).
This seems to work very well for car forums.
1) You don't have to be a sponsor member to use the forum, but many of those who use the site a lot will feel charitable enough to donate. (which, with the price of bandwidth these days [cheap], should be enough to keep the site running.)
2) It is a one-time fee, so there is no monthly or weekly or yearly tracking to worry about.
3) The site can give away a sponsor membership as a prize in contests or etc, and it costs nothing.
Another $$ generation technique along the same lines is to invite vendors to the forum. There would be a vendor forum where they can post sales, respond to questions about their products, etc. The vendor would pay a monthly or yearly membership fee to the web site for the priveledge of being able to advertise and interact w/their customers.
I think either of the above situations would create a win-win situation for the forum patrons.