With a bit of difficulty in my opinion. I think that a forum makes a good addition to a content site but it is difficult to make too much £ from a stand alone forum. (Someone tell me I'm wrong!) Notting
The only ways I have made decent money with a forum was with subscriptions and affiliates. Having paid members (I only had paid members) meant I was going to get money just because people wanted access to the forum. The other way was using affiliate programs, but not the standard ones. I found products and services that were related to the forum, and made a deal with the person/company to give my users a special price. The site offering the product or service would set up an order page just for my members and each member that bought it, I got a kick back. This worked fairly well since the members got something at a discount and the company sold more. The only problem is, the kick back isn't very high since they company has to cut the price for my members.
Yeap, there was even a publisher website thing that allowed you to put a banner in your signature and you got paid for CPM. I think it shut down as of too little popularity and the lack of owner interest. I think it had potential though.
Donating works too on forums. Messages like "Keep the site Ads free" and "We need your support", will help get some donations. Also use google search engine that should bring some $.
A lot of people make money just with their signature in forums. It's just an additional way to get traffic and/or direct sales.
How you can make lots of money off forums is if you develope it and then sell it off.. I've seen quite a few who have done this.
I think all web site are hard to market ( except for porn ). Persistence and dedication is what sets the failures and those who succeed.