This is the common question for Google AdSense newbie. I would like to share one of the possible reason on why your AdSense keep showing PSA and did not generate any cents for you. You can check it out for your self, it maybe due to occurrence of AdSense "Stop Words" in your website. In order to achieve search engine optimization, you must avoid using any "Stop Words".
What is AdSense Stop Words?Stop Words is a collection of bad keywords that sometimes called poison words which MAY prevent normal Adsense ads from being shown. These reputed stop-words cause PSAs (Public Service Ads) to be substituted, instead of normal AdWords ads. PSAs do not pay any money, but count as impressions, thereby diluting your AdSense statistics, and degrading your search engine marketing efforts.
Check out Adsense Stop Words via Vaughn's Summaries