I have been using Google Adsense for years, about 5 of them actually. I have seen the effectiveness and CTR slowly decline over the year. Damn near halted. I have seen a few people displaying a newer style of ads from a company called Chitika. These ads are relevant to the search term people used to get to your website. So if people were searching for car parts, found your blog, the ads they are shown, would be car parts =] They are like a different variation to the ever popular adsense.
So if you have adspace to fill, try incorporating Chitika into your content and see what happens. My CTR rate is dynamite compare to google. I think people are blind to adsense these days and Chitika is a fresh new change.