Monetizing a forum is a real challenge. This issue was discussed in the Financial Side of Running a Forum [webmasterworld.com] thread, but it's probably good to have a discussion specifically about the revenue side.
Memberships are one approach, but require a well-established forum with real perceived value. It's kind of like HBO - with so much free (ad-supported) programming available, to earn their premium fee they really have to offer unique content that people want.
Forum sponsorship is another approach. If you are in a topic area that has some solid advertisers, consider getting out of the CPM/CPC banner rat race and doing one or more negotiated sponsorships. This might look better than cheesy banner ads, and would be a more prestigious placement for the sponsor(s). It's a win-win, IMO, if both you and the sponsor are quality operations - your affiliation will help each other. If the forum is very popular, you could even have individual sponsors for your major topics.
One danger, of course, is that if a sponsor is a key player in your topic area they may well be a subject of discussion. You'll have to ask yourself in advance how to deal with topics like, "Widgetco Defrauded Me!", "Widgetco Has Lousy Customer Service", or even weak negatives like, "Is Widgetco Slipping?" If you edit or remove posts, even if those posts would have been edited/removed had they discussed ANY company, you'll get the accusation that you are playing favorites or are a tool of Widgetco's PR department.
As in most aspects of forum management, if you are up front about policies and consistent in their application, you'll weather the occasional storm.