WebmasterWorld has a featured thread on AdSense and Discussion Forums named 6 Tips for Adsense on Forums. In the thread member vabtz gives six valuable tips. (1) If the topic of your forum is targeting high CPC keyword then you will make a lot more money. (2) Ensure your pages are crawlable. (3) Don't show ads to registered users (users logged in), they won't click anyway. (4) Try positioning the ads until you get the highest CTR.
For the other tips, check out the thread.
I personally like how DigitalPoint configured his AdSense on his forum. One smart method is to use AdLinks within the footer of each post that the user is looking at. For example, if you go directly to a thread at http://forums.digitalpoint.com/showthread.php?t=26766 the Google AdSense AdLinks are directly under the first post. But if you go to a specific post at http://forums.digitalpoint.com/showthread.php?p=299674#post299674 you will see the Ad Link ads dynamically show up under that post. Very clean, simple and tactful.