Forums are notoriously difficult to convert with Adsense so these tips might be quite useful. They’ve included with the tip a useful heat map which points out the ideal positions from their experience. As per usual the top left hand sidebar area is the hottest with horizontal ad units between the first two posts also being important.
They also advise allowing image ads.
The only other tip that I’d add to their list would be to try rotating the colors of your ads. This is a strategy that attempts to overcome ad blindness from repeat readers which is a big problem for forums.