The following is such a common misconception that I actually believed it for awhile (even though it's my job to know this!)

Keep old lines of credit open
Closing old accounts shortens your credit history and lowers your score. Lenders also take into account the average age of your accounts, so an older account can help balance newer credit. Source: SmartMoney.com

Closing an old account DOES NOT shorten your credit history. The length of time you've had credit is not affected by whether your oldest account is open or closed.

And while an older account may help balance newer credit, it does not matter whether that account is open or closed.

However, it is true that closing an old account could lower your credit score a few points by increasing the balance to credit limit ratio on your open accounts.

But I would close an old account anyway, because it's a hassle to monitor a credit card account you never use. Here are some of the things you need to keep in mind if you keep the old account open:

1) If the account is open, it's possible for someone to use your credit card fraudulently.

2) You are more likely to forget to make a credit card payment for a card you think you never use, and thus severely hurt your credit score.

3) You need to make sure to update your mailing address for this account every time you move. (You need to know if you're being billed for some reason, and this also helps avoid fraud.)