First, start monitoring your credit card statement (thank goodness for online banking) for unauthorized activity--though, given that the breach has been happening for a while--you may have already noticed something awry. Look for charges for items or locations you don't recognize.

You can also place a fraud alert on your credit report (through ) if you want. Here's how Experian explains these alerts: "Fraud alert messages notify potential credit grantors to verify your identification before extending credit in your name in case someone is using your information without your consent."

Finally, remember that both and MasterCard offer what the companies call "zero liability" to the shopper for unauthorized credit card purchases.