A recent FICO study shows that an estimated 24 million Americans with no change in income and no late pays on their accounts had their credit lines reduced between October 2008 and April 2009.

This can be a real double-whammy. If you're traveling, you're inconvenienced because your card may be denied for a hotel or car rental. And then, of course, the credit line reduction can reduce your overall credit score. And it's all because of an arbitrary decision made by some giant monster mega-bank!

The FICO study also shows that of these 24 million people, 50% of them had very high credit scores of 760 or greater. So the banks, in a rush to reduce their own level of risk, went after the wrong people!

Remember, your available credit use accounts for 30% of your total credit score.

If, for example, you have a line of $20,000 and owe $5,000, then you are using 25% of your available credit, which is perfectly acceptable. But if your card is cut to $6,000, suddenly you're using 83% of your available credit, which is a no-no.

According to FICO research, people using 70% or more of their available credit are up to 50 times more likely to default.

Here's what you need to know: If you're hit with a sudden credit line cut, go look for another card if your credit is still good enough. See several popular cash-back options.

And if you only have a single card or 2 cards, be sure to use these "back of the wallets" every 6 months to keep them active in your credit mix.