EXTRA: Why does my credit stink?

You pay your bills on time, you don't have a ton of debt, and you don't make any unnecessary purchases. And yet, your credit is so mediocre. Why is that?

For as important as a credit score is to one's finances, most Americans know very little about the mysterious formula that produces the three-digit numbers.

Related Story: What you need to know about the "Credit Card Bill of Rights"

"I don't understand why I have bad credit," said Tampa resident Lindsay Poulin. "I've overpaid on credit cards if I can, I've paid them off if I can, I've never missed rent, bills, nothing... and I can't seem to get my score high."

Poulin, 27, says poor scores have kept her from receiving loans and credit increases in the past.

"I think I'm doing all the right things, but clearly I'm not," she said.

10 Connects put Poulin in-touch with credit expert Laurie Zoock, who pointed out that her score was suffering because of too many open credit cards. She had about 10 of them.

"Don't get sucked into opening multiple credit cards because you're going to get 10 percent off on a one-day purchase," said Zoock, who isn't an attorney, but gives advice on credit issues.

"Ultimately, a lender is looking at the predictability of you paying," she said, adding that credit scores are more a reflection of future risk rather than previous history.

According to the Fair Isaac Corporation, the company that determines the FICO credit scores, a person's score is made up of five types of data:

• Payment history (35%)
• Amounts owed (30%)
• Length of credit history (15%)
• New credit (10%)
• Types of credit used (10%)

MyFICO.com offers tips on ways to improve your score, but Zoock says there are other secrets she recommends.

"The research I've done shows that just having a couple of credit cards with the higher limits (is best for your score)," she said. "But you want to make sure you can pay those credit cards down - in my opinion - to less than 25%."

Other simple pieces of advice Zoock included:

• Prioritize paying down revolving credit (credit cards) over installment loans (mortgage, car, student loans)... but don't even miss payments!
• If you've missed a single payment on a card, you can ask (in writing) for a "goodwill adjustment" from the lender to forgive the one-time mistake.
• Inspect your report and investigate anything that looks like it may not be yours. Dispute any inaccuracies.
• Be selective when it comes to letting lenders, brokers, or companies pull your credit.

"Every time you give your name and social security to somebody, you're giving permission... to pull your credit and it's going to take points off your score," she said.

While every person's credit situation is different, Zoock says someone like Lindsay might want to consider closing some of her inactive credit card accounts. However, while it could produce positive results in 6-12 months, it would likely cause her score to drop in the short-term, so it is not something that would benefit someone who was looking to make a major purchase this year.

Many people, she says, are just fine letting their old accounts expire. For personalized service, she recommends talking to an expert.

However, both FICO and Zoock strongly encourage you at least glance at your credit reports once a year. And even if you don't anticipate any major purchases anytime soon, now is the best time to try and improve your score.

A few credit points could save you a fraction of an interest point on a loan, which could save you tens of thousands of dollars over the years.

For more education on credit scores, visit MyFICO.com.

For Laurie Zoock's credit services, visit Creditadvocateservices.com.

For more information on your personal credit report, visit annualcreditreport.com (note - freecreditreport.com is a subscription service that provides an estimated score, not the real thing!).

For more information, courtesy of Laurie Zoock, click on the following: (PDF)

• Credit Score Fact or Fiction
• Credit Quiz
• Outrageous Credit Facts