I recently came across a blog by momscashblog regarding how to optimize keywords for Adsense in order to receive higher payout.  (The post states she received information from another blog called Adsense Addict)

Here is a quick summary:

-  Use the Google Adwords keyword tool to find a list of similar keywords.  You want those keywords with high advertiser competition.

-  Check out the payouts for each phrases.

I played around with it a little bit, and it seems like most of the payouts are fairly low.  However, there are areas where the payouts is higher than others such as credit cards, web hosting solutions.  (Note:  These generally offers large amount of kick backs for affiliate marketing traffics too).

It is definitely an article that is worth reading.