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My posting at [webmasterworld.com...] gives some helpful tips on how to get AdSense working better for you, and this is especially relevant with phpBB, which isn't written very well for search engines and spiders to go through.
It's important that a thread about, say, mobile phone costs are accompanies by mobile phone ads, and a thread about how to boil a chicken gets accompanied by chicken boiler ads: to achieve this, you essentially need to re-write most of the phpBB templates to make them easier for the spider. This is a lot of work.
One thing I have done is obfuscate the mention of "phpBB" in the forums; both to 'hide' my phpBB forums from recent worms, but also to stop ads for phpBB hosting appearing, which were inappropriate to the subject matter.
I've also placed an ad placement position WITHIN the forum posts, so that they're in the content that people read. (Again, this is a non-public hack I've written). This does better in terms of click-thru, especially if the content is relevant to the subject matter.
Hope that's helpful.
I've also changed all the page names from .php to something else (an abbreviation of my site name, actually) so that the worms won't realise it's phpBB any more - but that's got nothing to do with AdSense.
(Although - actually - if it was changed from viewtopic.php to viewtopic.widget I wonder if it would get more ads relevant to widgets? Hmm.)