I don't earn my living on AdSense, but I make enough to pay most of the costs associated with about forty domains that I have sites on. Since I depend on people clicking on my AdSense banners, I make a point of clicking on others every now and then, if I am on someone's site that I enjoy or have gotten something from. After all, someone has to pay for this stuff. Usually, the ones I click on are something that I have at least a vague interest in, but I have been known to click on an ad just because I thought the site deserved it.
On my own sites, I usually use only one AdSense block. In that way, my site doesn't appear cluttered, and if you display only one ad on a page, the one that will be displayed is the one that has bid the most for the spot. Thus, it is the one that you will earn the most from. It seems like a win-win to me, and I know that I appreciate it when people click on mine, so I tend to reciprocate.