There is no such thing as a universal solution for ad placement. You won't find it here and you'll never find it. I rather write this article to give you, ad Publisher, some general guidelines on what to try and how to try it. After reading several blog articles on Adsense Ad Placement and how they doubled or tripled their income overnight and trying every Ad placement they suggested, I still had a terribly low CTR and income for the number of visits I was having. I suppose you know exactly how I felt.
The solution for my extremely low CTR was not using a particular ad unit nor placing ads above the fold, one of the first recommendations you'll find on the web. The key for me was to start using my head.
Where do your users spend the most time?
This is the key point. Take a look at your site, at your very best posts, and try to figure out where do your users will most certainly pause, or slow down. It's not hard. Besides, who knows your site better than you do? Just remember most of the users, specially the ones coming from search engines, are going to start a scrolling frenzy as soon as the scroll bar appears.
Ask yourself: if I were searching for X (your topic/keyword) and I were scrolling down this page, where would I slow down? Do most posts have a key image or text near the beggining of the page? Are many users getting to the bottom of the page? Are many users scrolling down as soon as the page loads? You should have a good idea by the way you write, the quality of your content, the source of your traffic, the number of comments you are getting, your bounce rate, etc.
In my case, I was surprised to find out that placing one ad close to the fold (not entirely visible in most screens) and one below the fold performed twice as good as having two ads above the fold. What was happening? My users were inmediatly scrolling down to the content, completely hidding my ads. I was pushing my ads to the top, away from where my users spent the most time. After I moved my ads closer to where I would slow down, the results were immeadiate. I guessed right.
That was still just a first guess. I kept making small changes and tracking performance and my CTR has improved accordingly. If you are into maximizing your income, keep perfecting your ad placement until you feel you are getting the most for your content and traffic. Make one change at a time and be patient. Wait for enough time to draw conclusions.
Sidebar vs in-content ads
Once you know were your users pause, there's another tough decision to make. Should you place the ads in the sidebar or mixed in with your content? If all you want to do is earn the most, in-content ads deffinetely perform better. The more unusually placed your ads are, the more they'll stand out and the higher your CTR will be.
According to the above heatmap, released by Google a while ago, ads perform better at the left than at the right of the viewport. My guess is that it's precisely because they are usually placed at the right, while left sidebars are traditionally used for navigation menus. Ad blindness is very real. Google has stopped using the heatmap, but I still find it informative, mainly because it proves that idea.
Make your ads stand out, without confusing your users, and you'll have happy users, high CTR and a good-looking site. On the other hand, in-content ads are a bit more intrusive than sidebar ads, though most users are used to them by now.
Recommended Ad Formats
Google Adsense has two recommended ad formats, or most succesfull ad sizes: the 336x280 Large Rectangle, the 300x250 Medium Rectangle, the 728x90 Leaderboard, and the 160x600 Wide Skyscraper. Most of the Publisher opinions I've heard say these ads do offer the best performance, and that has been my experience too. If they fit nicely into your layout, by all means, use them. The reason they perform better is that those are the most common formats among Advertisers, which means there is higher competition for those sizes. By using recommended formats, you increase your chances of getting the best ads. Good ads make both your CTR and Cost Per Clic increase.
The Medium Rectangle is great for responsive layouts because it's fully visible in all devices without resizing. My own experience is that it performs far better than the Responsive Ad Code.
Getting more Serious with Ad Placement
If you are living from online ads it certainly pays to take ad placement more seriously. An even more informed way to plan your ad placement experiments is to use a heatmap service. There are quite a few free and paid heatmap applications which generate scroll and click heatmaps, showing exactly what your users are doing.
Most free heatmap services don't have good user reviews and scores. However paid services like CrazyEgg start at $10 per month and most offer free trials. If you are making big money from online Ads, it's most certainly worth it.
However, heatmaps are just a tool to make the guessing easier. Experimenting with ad placement is the only way to really know if your ideas work. Don't let others tell you where to place your ads. Even if you think you are doing fine, you could still be losing money. Do small changes and track them for enough time. In most cases a few days are enough to notice a difference. You may be surprised by the results.