The Credit CARD Act did away with the worst of the late-payment abuses, but consumers still need to be proactive. Now, issuers can charge you $25 if you pay late, then increase that to $35 if you make another late payment within six billing cycles of the first one. Keep in mind that making a partial payment or failing to pay the minimum -- even if only by pennies -- is enough to trigger the fee.

In addition, if the payment date falls on a Sunday or a holiday and you pay electronically, you might want to think twice about waiting until the last minute to pay, says Pamela Banks, senior policy counsel for financial services at Consumers Union. Most big issuers process payments every day of the week, but if yours won't, it will hit you with the fee.

Avoid this by paying a day or two early, says Banks. Better yet, schedule an automatic online payment a few days before the due date. That way, if you don't get the confirmation email or if you don't see your balance change, you still have some time to sort things out.