5. Bouncing checks

Overdrafts won't show up on your credit reports or affect your scores unless you fail to make good on your bounced transactions. If you can't cover the debt, though, the bank can turn you over to collections and that can hurt your credit. Plus, you're likely to wind up on one of the blacklists the banking industry uses to keep track of those who've abused their checking accounts. That can make it tough to open another bank account for years to come.

If you're having trouble keeping track of your bank balance, you should:

  • Sign up for email or text alerts that will tell you when you're running on fumes.
  • Turn off "bounce protection" or "courtesy overdraft." Banks have been pushing this service hard because it rakes in lots of profits.
  • Consider true overdraft protection, which links your checking account to a savings account, credit card or line of credit. This is the best choice for occasional overdrafters. If you're chronically running on fumes and in danger of racking up debts you can't pay back, just shut off bounce protection and learn to live with the money you've got.

6. Carrying a small credit card balance

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Carrying a balance on your credit cards is financially foolish, but as long as the balance is small, you won't hurt your credit.

But small means small, as in less than 10% of your available credit. Any balance that eats up more of your limit can start to dampen your scores. The bigger the percentage of your limit you're using on any card, the worse the potential impact.

You also should know, though, that carrying a balance doesn't help your credit. Having and using your credit cards lightly are what's important -- you don't need to pay interest to boost your credit scores.

7. Getting a student loan deferral or forbearance

Skipping a payment on a loan is a huge, hairy deal, as I mentioned above. But if you make formal arrangements with a student lender to enter a deferral or forbearance program, your credit shouldn't be affected.

Just make sure you're approved for the program before you stop making payments, since a misunderstanding could cost your credit scores dearly. And use these programs sparingly, since your debt will typically grow when you're not paying it down.

Also, understand that student loan deferral and forbearance programs are unique. Other workout or loan modification programs, such as a mortgage modification, may affect your credit scores.

8. Having a fat wad of credit cards

Keeping track of a bunch of credit card accounts can be a pain, with all the different due dates and potential fees you might have to pay. But rest assured: There's no such thing in the FICO scoring formula as having "too much available credit." Open credit lines generally have a positive impact on your scores, which is why you don't want to close accounts or request lower limits if you're in score-improvement mode.

You don't want to rush out and open a bunch of new cards, though, since each application can ding your scores. But if the accounts are already open, you shouldn't worry that they're dragging down your scores unless (again) you're using too much of your available credit. If you are carrying big balances, spreading the debt over several cards and paying down those balances over time should help your credit; closing accounts will not.

9. Obsessing about your credit scores

You could check your credit reports and buy your credit scores every day for years, and still not impact your FICOs. That's because the FICO scoring formula ignores any inquiries you initiate through official channels such as the credit bureaus, AnnualCreditReport.com or myFICO. Where you can run into problems is in having your credit pulled by a lender or even a friend at the bank, because those typically will be coded as "hard inquiries" that can ding your scores.

Also, checking your credit a lot can cost you a lot. You get only one free peak at your credit reports each year, and you typically can't get your FICOs for free (if you're getting free scores, they're probably not FICOs, or you've signed up for credit monitoring that will cost you a monthly, quarterly or annual fee). Scores at myFICO cost $19.95 a pop, and the credit bureaus charge various fees to view your reports after you've exhausted your annual free peeks.

You're smart to review your credit reports once or twice a year to monitor for identity theft and errors. You also should check FICO scores a few months before you plan to apply for a major loan. Otherwise, though, you should probably relax and find a hobby that doesn't involve obsessing over three-digit numbers.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.