Your credit score helps determine how much you can borrow and the interest rates you'll pay. These sites explain some strategies for maintaining or increasing your credit score.
This video and article at MoneyTalksNews includes a laundry list of ways to clean up and improve your credit score. You get a better score by asking for and getting higher credit-card limits - and then paying down the balances. And if you've got generally good credit, but miss a couple of payments, it is possible to ask the credit company for a "good-will adjustment" - where the company actually removes a late payment from your credit report. You won't be offered this option; so you have to request it, this post says.