Placement of AdSense ads depends on whether you have a content site, blog or forum. Also the site layout like left sidebar, right sidebar or full width page decides the suitable ad sizes to be displayed in those pages. Besides using recommended ad sizes by Google, it is also important to place ads in appropriate places on each page of your site to increase the revenue from AdSense.
Type and Layout of Site
Whether it is a forum, blog or content site, each type of site has its own layout and theoretically deciding ad placement may not exactly suit all cases in practical. Nevertheless, most of the sites fall into a standard layout having header, sidebar, main content area and footer with minor modifications like right sidebar, left sidebar or full width page.
In general Google suggests placing ads for a home page and article page based on the layout.
Ad Placement Example Case 1
Place one unit 728 x 90 leaderboard ad either in the header area or just above the content.
Place the second unite 300 x 600 large skyscraper in the sidebar in a suitable place based on your average post length.
Place the third unit 728 x 90 in between the content or below the content depending on the post length.
AdSense Ad Placement Example 1
Ad Placement Example Case 2
Place a 336 x 280 large rectangle ad beside the content at the start of the article.
Place a 300 x 250 medium rectangle on the sidebar.
Place 336 x 280 large rectangle or 728 x 90 leaderboard below the content.
AdSense Ad Placement Example 2
Now that the limit of 3 content ads is reached and you are left with 3 more link unit ads and 2 search boxes. Important point on using both link unit and search box is that both reduces the coverage which means you will be paid only when the user click on ads on the results page and not just for searching or clicking on link unit as such. So we insist to use link units only when you have lot of space on the page after using 3 content ads.
Other Factors for Ad Placement
Ensure to select “Text and display ads” as “Ad type” so that both type of ads can be displayed on your site thus improving the possibility of higher revenue.
Check out the rating of your AdSense scorecard for revenue optimization which will show the rating of usage on recommended ads. You can change the ad formats to improve the rating.
Try out the experiment feature in AdSense account with different ad styles (for example with and without border) and choose the winner after a period of time.