Darren Rowse has posted about increasing Google Adseense advertisement relevancy on his blog Problogger.net.
He looks at things like Section Targeting, Metatags, Keywords, Content, Titles, Sidebars, Pictures and finally Contacting Adsense.
My favourite would have to be Section Targeting:
AdSense acknowledge the problem of irrelevant ads and have come up with a way of helping their ‚Äėbotsí determine what parts of your site you want the ads to relate to (section targeting).
In short – section targeting allows you to put code around the most important parts of your page (ie the bits that you want the ads to relate to) and also allows you to put code around parts of your page that you donít want AdSense to consider when serving up ads.
When I use section targeting on a blog page I generally put the code that highlights the important bits from just before my post titles to the end of posts. I also tell the bots to ignore my sidebars and footers.
Check out his full post at Problogger.net.