Since Heartland processes about 100 million payment transactions per month for 175,000 merchants, the chance that you've been affected is good, if you ever use your credit cards.

Heartland's president and CEO Robert Baldwin says the company plans to notify every card holder who was affected - once the final count has been determined. Since this could take some time, consumers are wise to take their own precautions. Experts are calling this breach the largest ever - even bigger than the 2005-2006 theft of 45 million card numbers from TJX.

According to the security firm, CardCops, the past year has seen a 20% increase in the number of hackers "testing" bunches of credit card numbers. These hackers send through a small charge - one you might notice and wonder about, but dismiss as unimportant because of the small amount. These charges are sent just to see if the account is active and some are refunded before showing up on your statement. Once the validity of the account is verified, they can sell the account numbers to those who will be the ultimate "users" of your credit.

What can you do? First, read every credit card statement and check for the validity of each and every charge. That means you should be diligent in keeping every charge slip and/or entering them into a bookkeeping program regularly, so that you don't have to rely on memory to tell you if you made a purchase on a specific day.

If you find a fraudulent charge, contact your credit card issuer to report it immediately. You won't be held responsible, but they won't know the charge is fraudulent and you won't get the charge reversed unless you report it.

Next, get a copy of your free credit report and read it carefully. Look for warning signs of identity theft, such as a new address or an account you don't recognize. While you're there, check for mistakes. According to FICO, one in four credit reports contains mistakes, and those mistakes could significantly lower your credit scores.

Then, sign up for a notification program - so that any time there's activity affecting your credit report, you'll know it.

As the economy worsens, thieves are finding more creative ways to access funds that don't belong to them. You can't prevent their activities, but you can stop them from destroying your credit scores and your financial reputation if you keep watch on your own credit report.