One of my boards got really out of control with bandwidth as well.
Ours is a "support" type of board...which discusses adult content. We started by seeking member donations through PayPal. You can add a "PayPal donate" button at the top of your pages.
Because the members loved our site so much.. they coughed up the dough in a big way. Some $5 some $100 it was incredible. However, each time we were past talking about the crisis, donations would dry up.
We had a couple of furms where members were posting photos. So about halfway each month... we would cry bandwidth troubles and close the photo forums to everyone but those who contributed (used usergroups to set up). This would start the cash flowing again.
Now we have moved the almost 7000 photos to PhotoPost and put it behind a password protected directory with iBill. Photos are paid access now, and funding the site quite nicely.
We still have our donation system in place. But now, those who donated get access to a couple of private forums we have set up. We give them an @ourdomain email address (we've even installed neomail and added a link from MyHome), and our supporters also get to choose their own custom title for their username.
So far this is working well.
The donations are tougher to keep up with, because you have to manually switch their usergroup when they donate. Also, people would send money via paypal, but the email used for that didn't match any emails in our system, so we had a tough time keeping track of who the money was from. And access for them to the member areas didn't happen until we changed the groups. The iBill setup is much easier... (if you've got something like photos to sell access for) and instant gratification for them.
