Forums are tough to earn from since most just come to read and disregard ads. Same reason why it's difficult to sell private ads deals as well. Perhaps you can try using the second tier referal programs for various networks and affiliate programs.
Selling private advertisements such as banners is hard on forums and really not good for either side as cost per 1000 impressions is next to nothing and neither side really gets much out of it when you could put affiliate advertising and google adsense on there and make some, don't expect to get rich off of it unless the board is HUGE. Also might look at a system that sells a few text links as I know many people who frequent your board will buy text links for a few dollars a month just because they want to see their domain name they own around the board.
Just had an idea, you can always try to monetize your members when they first sign up. Kind of like a nicer version of coregs, where you're making new board members aware of sponsored websites which match your forum theme.