1. Are you utilizing your ad spots?
This is the first questions you should be asking yourself. AdSense won't allow you to add as many ad units as you like. But are you even utilizing all the ad-spots that it does allow? For every page on your website, AdSense will only let you place three AdSense for content units. These vary in size, so you can utilize the best possible locations your ads can fit in. But you can only place one 300x600 ad unit. Apart from this, you can use three text-ad units, and two AdSense for for search boxes units. Before placing all these ad-units though, take some time to consider whether you have a sizable quality content to ad unit ratio. It just doesn't make sense if all the ad spots have been used, but there is no actual content on that page.
2. Take a look at your ad sizes
Generally speaking, wide ad units tend to perform better than tall ones. For example, the 728x90, 336x280, and 300x250 ad sizes are usually the highest performers. You should try on multiple ad sizes, and find out for yourself what works best for you. The ad size should also correspond to the placement of your content. For example, a tall and thin ad-unit doesn't make sense within a written post.
3. Use text and images too
Usually, publishers just focus on image ads, and forget about the other options available. Adding text ads is a good idea as well, because they compete with the image ad units for the same space. Using a combination of text and image ads can help you earn a lot more.
4. Add search boxes
As mentioned above, image ads are not the only thing in the world. AdSense will allow you two AdSense for search boxes units per page as well. They let you place a custom search engine on your site to provide users with web search. And you might earn from relevant ads on the search results page.
5. Make your ad units accessible
Pay special attention to where you actually place your ad units. They should complement, and fit in with your content. And also, users should be able to easily identify and access them. Generally, ads should be above-the-fold. Google offers a lot of guidelines to help you place your ads.
6. Check out color combinations
Last, but not the least, use trial and error, and play around with various color schemes and ad styles. They can play a big part in how much revenue you earn. There are three strategies you can choose from. Blend, Compliment, and contrast. Compliment is normal placement, where ad units are used within content, and are distinguishable. Blending ad units into content means using the same colors, so that it is hard to distinguish the ad unit. Contrast means you use contrasting colors that scream out an ad-unit's presence. You can use any of those, as long as you follow the guidelines from Google.