Holiday shopping can wreak havoc on your credit score, even if you're disciplined. It's easy to exceed the credit limit on your credit card and you may succumb to the temptation of opening up a retail credit card in order to score extra discounts. Always remember to pay your credit card bill. It sounds like a no-brainer, but missing just one payment could drop your score 100 points. Also, be sure to pay off your credit card bill in full. The interest expense you'll owe by leaving a balance on your card will likely erase any savings you scored from store discounts. MainStreet's Scott Gamm shares tips from New York.