How to control your credit rating

For more information on managing your largest investment, check out Gerri Willis' "Home Rich," now in bookstores.

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NEW YORK (CNNMoney.com) -- From buying insurance to getting a mortgage, your credit score is your financial DNA. Personal Finance Editor Gerri Willis is here with her dos and don'ts of establishing good credit.

If you are trying to improve credit scores, don't close credit card accounts. That's because your score takes into account the difference between what credit you have available to you and what you're using.

If you shut down a credit card account, the total amount of your available credit is lowered, and your balances look much larger in comparison. This ratio then hurts your score.

Keep your debt to utilization ratio to 30% of your credit card limit. Your FICO score also looks at how long you've been managing credit. The longer you've been managing credit, the better it will be for your score.

Forget those retail store credit cards.

Every time you open an account with a store to get that 10% discount, you are giving the retail lender the ability to pull your credit score. And that can lower your credit score. This is especially damaging if you've only handled credit for a limited time.

For example, the credit score of a 20 year old with only 1 or 2 credit cards will see their score drop more substantially than someone who is managing credit for 25 years.

Paying your bills on time is about one-third of your FICO score. However, you should concentrate not only on paying on time, but paying more than the minimum payments.

The amount of debt you have is also vital to your credit score.

"You can be in a crushing amount of credit card debt, but if you're only making minimum payments, you're losing points because your balances are slowly creeping toward your credit limit," says John Ulzheimer, of Credit.com.

To really improve your credit score, you should only spend within 10% of your credit limit. So, if you're credit limit is $6,000, don't charge over $600.

Most debts, except for bankruptcies, are erased after seven years. If you've had a foreclosure or have a few delinquent payments, you can still raise your credit score to above average.

The older this negative information is, the less important it is to your credit score. And just think, if you raise your credit score from 670 to 715 - and that's only 45 points, you'll save over $82,000 in total interest charges on a $200,000 30 year-fixed loan.