Ad Position and number of ad units per page

Ad Position Where to put the ads is another factor that affect your adsense revenue. Of course, the ad positions available on your pages are governed by your page layouts. However, if you don't have your site built yet, you may work from anothor way round; design your page layout to optimize your adsense revenue. Studies shows that the following positions are where users pay most attention to (and thus resulted in higher click rate) :

Top Left corner - this is where your site logo or your site navigation (veritical one) appear. Depending on whether you weight your site usability or your ad reveneu more, you may consider putting your ad unit here (below your logo maybe). There are sites doing so and put their site navigation on the right hand side.
Top of the page - this is usually where most ad banners appear
The space between your horizontal navigation bar and your main content - This position is where your users are looking for the information they want, so a location with good attention.
The space follow your main content - the reasonale is that after reading your main content, your user is looking for the next thing to move to, so ads at this location can have high click rate.

Number of Ad Units Depending on the length of your page, you can decide how many ad units to put on it. If you have a long page, you can consider putting ad units in the middle of your content. However, please be noted that more ad units don't necessarily yield higher revenue. According to Google, ads with higher CPC are having higher display priority. If your page has less ad units, in theory, most of the ads displayed would have higher pay. If your have more ad units, essentially, your are mixing higher pay ads with lower ones; which ad being clicked by your user is more or less by chance. So, displaying an ad unit in a prominent position and a link unit at the end of your content may be a good enough combination.