Most forum designs, unfortunately, are not designed to work well with Adsense. They tend to have fluid layouts which do not fit Google's fixed ad sizes well, and it is often difficult to blend the ads in such a way that it flows with the rest of the content on your page.
There may come a point when you have tested, tried, and incorporated every conceivable ad variation, style, and color on your website only to see marginal improvements. If this is the case, you may want to consider an entire redesign of your forum.
I ran into this problem with DebtManagementTalk.com. Since the site is a revenue sharing forum, it relies heavily on the ads performing well. Although Google's terms of use restrict me from saying what the clickthrough rate was on the site, I can say that it was lower than I had desired, and much lower than what the members on the site were looking for.
Through using the techniques I listed above, I was able to make small improvements to the ads performance, but the overall performance was still less than desirable. It was time for a redesign.
DebtManagementTalk.com moved from a fluid (full screen) layout to a fixed width layout. The colors were changed to be more vibrant and to offer more possible color options with the Adsense ads. The discussion layouts were stylized differently to allow for a much more natural incorporation of ads.
Although testing is still ongoing, the redesign improved the Adsense revenues on the forum significantly.
