If you're in the U.S., utilize the "safe harbor" provisions of the DMCA ...
Costs $30 USD to file an "Interim Designation of Agent to Receive Notification of Claimed Infringement" with the U.S. Copyright Office.
You will also need to create a webpage outlining the procedure for reporting alledged copyright violations, which contains, at minimum, the full name, postal address, phone number, and email address of the designated agent on file at the U.S. Copyright Office (ie. you or perhaps an attorney, etc) - the webpage should ideally be linked from every webpage containing user content.
Online Service Providers information page - instructions on how to register are here:
http://www.copyright.gov/onlinesp/
Directory of Agents currently registered:
http://www.copyright.gov/onlinesp/list/index.html
Ron