Found this by mistake as it was a coding error, but had a slight uptick in CTR for AdSense. Insert your Adsense code inside a normal div layer, same size as the google ad itself. Then set the background of the div layer to be DIFFERENT to the background colour of the AdSense advert. The page will load with the coloured div layer - then the Google ad will load into the div. You get a slight flash where the ad banner appears - the length of the flash depends on your own pc bandwidth.
Not allowed to say It was slight - but it all helps. I have actually disabled it - as my personal sites are aimed at long term regular users (and it can be slightly annoying), but for one off visitor websites - this could be a useful aid.