To increase/improve AdSense CTR you can spend all the time in the world following other people advices or you can find that out on your own! Remember that whatever works for someone will not always work for you. Start by placing ads near hot spots on your website, if you run a popular blog and you have a lot of replies place and Ad unit near reply button, you have to identify hot spots and put AdSense there, leave it there for a few days then try another location, after a few days compare those location CTR. The common mistake people make is they look for "The Best Performing Banner Sizes for AdSense" but you need to use an Ad size that is most appropriate for your website! You have to make those ads as discreet as possible, what is the point to have AdSense 300x600 Ad unit (that has the highest CTR) when you can it clearly a mile away!