Use the default ad unit
The default 300x250 ad unit will make you the most money. There is a reason that Google has set this as the default size... it converts better than any other ad unit. Use it and love it.
Only put one ad unit on your page
Having more than one ad unit on the page can actually lower your earnings. This might seem counter-intuitive but it's true. Google puts the ads with the highest CPC in the unit closest to the top of the page. Even in a high-paying niche, ad units farther down on the page will often earn you only pennies per click. Amazingly, people will actually scroll back up to click on the ads when they've finished skimming your page.
Place the ad unit directly above your content
This one is a no-brainer. Placing your AdSense unit left-aligned, directly above your content, above the fold will get the best CTR. People love shortcuts. If you give your visitors a solution to their problem before they even scroll down, many will take it.
But don't just trust me, split-test different layouts for yourself and see how much money you've been missing out on. Hope this helps!