These tags will emphasize to Adsense what portion of your page Adsense should use to find relevant ads:

<!– google_ad_section_start –>
GOOD CONTENT HERE
<!– google_ad_section_end –>

You can also block off sections that you do not want to be picked up by Adsense by adding a (weight=ignore) to the start tag:

<!– google_ad_section_start(weight=ignore) –>
BAD CONTENT HERE
<!– google_ad_section_end –>

This is handy for blocking out headers, footers, sidebars, menus or whatever portions of your site don;t use your desired keywords. This can also come in handy when you post something on a blog that all of sudden changes all your ads to something wacky.

Be careful though – if you don’t provide enough content within the tags you will actual lose relevancy and may even get stuck with Public Service Ads.

Read more on section targeting from Google

If these tags are working for you – here is a great post to read, it’s a few years old but all the tips are still valid ways you can use to increase ad relevancy on your site.