3. Charging Interest on a $0 Balance

A credit card's so-called "grace period" is the 20 or 25 days between when the card company issues your monthly bill and the actual due date for payment. But don't be fooled into thinking that this window is a break from charging you interest. Most credit-card companies continue to charge interest on your entire balance during that time.

This trick, called "residual interest," is decidedly ungraceful for credit-card users who make large purchases and pay them off over several months. Why? Because they are charged interest on their credit-card balance even in the month after they've paid it down to $0, Sherry says.

Here's an example: Say you buy a $900 refrigerator that you pay off in three months, paying $300 plus whatever interest charge you have each month. Then on the fourth month, after you've supposedly paid off the balance, you still receive a bill with an interest charge. That's because the credit-card company has continued charging you interest during your third grace period, or the 20 or so days between receiving your third statement and the date you send in your payment, Sherry explains. "People tend to pay their bills on the due date," she says. "This is opening them up to 20 days of interest charges."

Is this practice common? You bet. "I was amazed to find that 50% of the banks I surveyed use it," Sherry says.