Credit scores typically look at your debt-to-credit limit ratio or "utilization" in two ways: They compare the balance on one revolving account to your available credit from that lender. For instance, if you have a credit card with a $1,000 balance and a $5,000 credit limit, this ratio would be 20 percent.

Scoring formulas also look at your debt-to-credit limit ratio a second way: calculating the total of all your debts on revolving accounts against your total credit lines on those same accounts.

So if you have four credit cards each with a $5,000 credit line ($20,000 in credit), and you have a $1,000 balance on two of them and nothing on the other two ($2,000 in debt), this ratio would be 10 percent.

"In an ideal world, you would want to have (those ratios) under 10 percent," says Hendricks. "But certainly you want to keep them under 40 percent. There's no magic number."

But if you're running up a balance of $2,000 to $3,000 with a card that has a $5,000 limit, "that's really going to hurt your score," says Brobeck. "And what's worse is running up balances on several cards."