Content is a big deal.

Google AdSense delivers ads to your site's viewers by analyzing the content of your site. In other words, it is contextual advertising. With that in mind, you will not get any advertisements related to your site unless you write specific content. For instance, on this site, you will often see ads for AdSense related companies. Also, if your site is mainly imaged based, there will not be enough text for your site to have AdSense display any content at all, besides "generic" advertisements.

Topical Site Content.

Some sites will be making a lot of money, and others will not, even though they may have a very similar layout and the same amount of text. The reason one is making more dollars could be the topic of the "winning" site. Certain topics will automatically bring higher traffic than other topics. In many cases, having a broad topic that is covered in many other websites provides a low payout, due to the fact that the market is saturated. Having a site focusing on a specific niche can oftentimes make a lot of money.

Quality Over Quantity.

Ok, maybe the heading should be Quality and Quantity. Having informative and high quality content that users cannot find elsewhere will drive your traffic through the roof. If users cannot find your information anywhere else, they will come back again and again to your site. Of course, having a lot of this high quality content will bring users back even more.

Update Your Site.

Frequent updating will update RSS readers, and give you more opportunities to submit your entries to blog catalogs and social bookmarking sites. This in turn will cause your website traffic to increase. More traffic equals more exposure to your advertisements.