There is a nice article on AdSense published recently on DevShed. For my own note I had no idea you could direct what content you wanted Google to concentrate on for displaying relevant ads.
Put the main content inside these comment tags:
<!-- google_ad_section_start -->
<!-- google_ad_section_end -->
And things you want ignored inside:
<!-- google_ad_section_start(weight=ignore) -->
<!-- google_ad_section_end -->
