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Many people don't even know how long they've been in credit card debt. Recent research from CreditCards.com found that 7 percent of plastic holders don't remember how many years they've been carrying a balance, while more than half say it has been more than a year that they've done so. When you carry a balance on your credit card, interest piles up and you end up paying much more for items than if you'd used cash. Doing so can also bring down your credit rating, said Ted Rossman, an industry analyst at CreditCards.com.

"A big part of how much you owe is your credit utilization ratio – that is, how much credit you're using divided by how much you have available to you," Rossman said, adding that people should keep their usage below 30 percent. "If you have, let's say, $5,700 of credit card debt, even if you have a pretty generous credit limit of $10,000, you're still at a 57 percent utilization rate which is higher than recommended and would hurt your credit score," he said. Many other moves can lower your score, experts say.

Paying off an old debt

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It seems like you're doing something good: An old, unpaid bill resurfaces and you make a partial payment on it. However, sometimes these debts are already so old they're not legally enforceable, said Katherine Lucas McKay, who focuses on consumer debt at the Aspen Institute. Once the debt collector makes a record of your active payments, it's legal in many states for the debt to be treated as new, Lucas McKay said. Often the easiest thing to do, she said, is to pay off the balance completely. "Your score will start to rise again once that record is closed," she said. If you believe the collection agency is acting inappropriately, file a public complaint through the Bureau of Consumer Financial Protection. For the debt collectors, she said, it's often easier to just resolve the issue "rather than have their regulator look into the situation."

Closing a line of credit

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After you've paid off a credit card, you might close the line of credit. Keep in mind that doing so will lower your overall credit limit, and therefore may increase your utilization rate — how much credit you're using versus what's available to you. "There is some data from FICO that reveals people with credit scores above 800 commonly maintain a credit ratio below 10 percent," said Bruce McClary, vice president of communications at the National Foundation of Credit Counseling. To offset the smaller credit limit, you should have a strategy for paying down the remaining balances as quickly as possible and bringing the utilization ratio back within an acceptable range, McClary said.

Sticking with one type of credit

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Ten percent of your score is about the variety of your debt, said Matt Schulz, credit expert at CompareCards.com. "If you've had a car loan, a personal loan, a credit card and a mortgage and handled them all well, you're probably going to have a higher score than someone who has just had a credit card — all other things being equal," Schulz said. That's because lenders have more data points to pull from to make their decisions. You shouldn't go out and get a loan you don't want or need just to bump your score a bit, he said, but, "it is worth considering in some circumstances."

Failing to check your credit report