AdSence seems to be not very good solution for forums as you got low CTR on such type of sites, I've got check from Google time after time and that's nice addition to family budget but it's not steady so I'm currently looking for alternative companies which provide CPM banners instead like
AdBrite which would give you more control over banners rotated on the site. AdSence seems to have problems sometimes with detecting right ads for the particular topic, marking content with adsence tags help a bit but not always, AdBrite from other side let you choose ads specific to your site but if you have very general community using them may be tricky as you'll have to make a zone for each board to cover each topic with relevant ads.
Also you may consider using intext ads which would analyze content of each thread and later highlight some words with related ads, it may compliment AdSence actually. I'm using
Kontera as although on registration form they ask for high amount of traffic they accept sites with smaller traffic numbers also.
So you may try them out on your forums.
But the main thing in choosing ads is to think about your members, I doesn't display ads for a long time on the forum just to let visitors to have some rest from it, but later I have to change this so now I've got few ads on the pages viewed by guests and members which have just registered, but members who stick with our community will see ad free forum which is a fair deal IMHO.