If you have never heard of CrazyEgg before, just visit http://crazyegg.com. CrazyEgg helps you tracking your visitors' clicks.
So, you can check, which adsense units get clicks, which headlines are more effictive, do visitors click links and sidebar and so on. After a couple of days using CrazyEgg i've noticed, that for one of my blogs top ad unit is square rectangle at the bottom of the page - people usually scan the front page through and then click "Older Posts" button. That's not a rule, for some of my blogs this unit isn't good.
Anyway, check CrazyEgg - you can have a free trial, and it's one of the most useful web-tools i discovered lately.