'Hints' on Google Hints
This subject was popular a couple of years ago and it is recieving attention again. An undocumented AdSense variable, google_hints, can be used to add a comma delimited list of keywords to your AdSense code which will offer 'hints' to Google about what ads should be displayed.
Sounds great. Why isn't everyone doing this? Because it is by invitation only. Adding this variable to your AdSense code is a violation of Google's terms of service and can result in having your AdSense account locked, or you could be banned from the program. Hardly worth the risk.
So, read all you like about google_hints, but be suspicious of any source of information that suggests you implement it!