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Making only the minimum payment
The rationale: As long as you don't have to pay more, you'd rather save those extra bucks.
The rebuttal: By paying only the minimum on your credit card bill, you're incurring much higher finance charges and increasing the real cost of whatever you purchased exponentially.
Say you have a $4,000 balance on a card that requires a minimum payment of 2%, and your interest rate is 18%. It would take you 428 months (that's almost 36 years!) and $10,397.20 in interest to pay off that debt. But if you added just $20 more to that minimum and committed to making fixed payments of $100 a month, the balance would be gone in 62 months and the interest shaved down to $2,154.49.
Bankrate's credit card calculator computes the payment schedule when you plug in your own real numbers.