Click Fraud: A Real Threat
The most important commandment for an AdSense publisher to remember is Thou shalt not click ads on thine own site. Clicking Google-supplied advertisements displayed on any of the sites you own to artificially inflate your AdSense earnings is a clear instance of click fraud.
Google's chief financial officer recently stated that click fraud threatens Google's business model. Most of Google's revenues derive from its advertising programs, so anything that threatens those programs is a threat to Google's long-term stability. Thus, Google takes a dim view of AdSense publishers who attempt click fraud. If you're caught doing it, you'll be kicked out of the AdSense program. So be careful with your mouse.
Note that the AdSense program policies also forbid publishers from enticing or otherwise encouraging visitors to click on advertisements. Visitors should click on ads out of natural curiosity, not at your urging.