joined:May 1, 2005
I'm always tinkering with things on my sites and generally I'm not sure it does much good. However a couple of months ago I noticed a little message from the Google elves suggesting I use more image ads as I may be missing out. Usually I ignore most of what they say ("you haven't got enough ad units on your page")
So I changed some pages' 336x280 text ads to be text/image ads. The result on two pages that I did this was that my cpc has doubled on one page and quadrupled on the other. I know I have to be vague here but let's say cpc was originally around the equivalent of $1. As these were two of my best paying pages anyway, you can imagine this has made a big difference to my overall earnings.
I now most of the time see image ads from an advertiser who is regularly doing tv ads, and so i guess they have a big budget. They don't seem to use text ads at all.
So just to say - you may do lots of tinkering and think you're wasting your time, but it may be that one change just happens to be the thing that makes a difference for your site in your niche.