As you probably know, Adsense ads are run as an auction. The highest paying person get the first ad slot, and then the next person gets slot #2, etc.
So the first ad on the page will typically pay higher than the next slot, etc, all the way down to the last ad on the page. You ideally want to get that first ad slot as prominent as possible, and get the most people you can to click on it.
Things get a little more interesting as well. Advertisers prefer different sizes of ads. So there might be more competition for leader boards, or skyscrapers. It also changes from industry to industry. I discovered that advertisers love skyscrapers in my industry.
This gives you two variables that you want to test.
1. Figure out which ad size is most popular on your website. You'll want to test different ads as the first occurrence in your HTML. This might break your site design a bit as you test this out.
Hopefully you'll discover which ad gives you the highest payout.
2. Figure out which ad converts highest on your site. I've found that the big square ads work best on my site.
Then put the information you've discovered together. Get the highest paying ad in the first position in your website, followed by the next highest paying ad. You might need to change your design a bit to make it fit, or reorder the ads on your site.
If you're lucky, you'll be able to significantly increase your eCPM. It made a big difference for me.