Google Ad Sense pays per view and per click... true the per view rate can very VERY low but even 1 or 2 clicks can quickly add up.
ContextWeb is another ad serving service which pays better than Google on a pay per view basis- but they can't fill every ad spot so you should still use Google as a backup.
You could also look into offering subscriptions for premium memberships- you can give paying members more options (more PM space, bigger avatars, etc..._)
You can also try
http://www.viglink.com/ or drivingrevenue.com, they will auto make any links anyone makes on your forum to participating online stores (amazon, ebay, many more) into affiliate links where you will get a percentage of any items they buy from your link.
Each alone may not bring in much but all 3 combined has been enough to pay for my hosting and licensing costs so far.