Do you use Google Analytics? By requiring your advertising partners to embed analytics into your ad, you can get valuable third-party information from Google.
Click fraud can also be deterred or prevented by using a third-party source such as Click Forensics or Click Hawk, but they're going to cost some money. Google Analytics is free.
http://www.clickforensics.com/http://www.trafficsentry.com/clickhawk/
You may also want to read this and other articles online about click fraud:
http://www.panalysis.com/prevent_click_fraud.php