The Web site of the Federal Trade Commission offers the following advice if your cards are lost or stolen:

• Report the loss or theft of your credit cards to the card issuers as quickly as possible. Many companies have toll-free numbers and 24-hour service to deal with such emergencies.
• Follow up your phone calls with a letter that includes your account number, when you noticed your card was missing, and the date you first reported the loss.
• If you report the loss before your credit cards are used, the card issuer cannot hold you responsible for any unauthorized charges. If a thief uses your cards before you report them missing, the most you will owe for unauthorized charges is $50 per card. If the loss involves your credit card number, but not the card itself, you have no liability for unauthorized use.
• After the loss, review your billing statements carefully. If they show any unauthorized charges, it's best to send a letter to the card issuer, describing each questionable charge.