Some website owners use wrong/ illegal tactics in order to increase their revenue from Google AdSense program. One example of misuse of Google AdSense program is websites that do not contain any useful information but, instead, have a lot of keyword based content that is targeted at fooling the search engines. Such websites are categorized as AdSense Farms. Sometimes website owners might use a large number of websites in order to create even bigger AdSense Farms that have completely useless content. However, this might lead to disqualification from the Google AdSense program. Another misuse of Google AdSense program is seen in the use of free content e.g. content from Wikipedia to attract traffic to the website. Google is always on the lookout for such websites and they keep upgrading their technology in order to bypass such websites. So, never use such methods that can get you barred from Google AdSense program. And remember, its difficult to outsmart Google on this front.
There are sufficient tricks and legal tactics available for getting more traffic to your website and for increasing the earning potential from your website. Use those tricks and track the performance of your websites in order to increase your revenues.