There are alot of factors that your credit score gets based on but the top six killers are:
1. High balance. - "Johnson estimates that you lose 1 point for every percent of your credit limit that you use. So if you have a total credit limit of $10,000 and have an outstanding balance of $4,000 (40%), your score would be 40 points lower than if you had a $0 balance. Ideally, credit experts say, you never want your balance to exceed 30 percent of your credit limit." (MSN.Com)
2. Late payments. - Pay on time! If by chance you can not pay it on time then pay in full, IF you can not do that the pay more than the minimum. I suggest setting up the automatic bill payments through your bank, you will never be late again!.
3. Not Enough Credit. - Banks like to see history and they can not judge how well or poorly you do with substantial credit. Its best to take out a car loan or bank loan and repay, on time. This will boost your score.
4. Length of Credit history. - Too young, too eager! You must build trust with the creditors. Start slow, start small and then build your foundation. It takes time and time on this matter is on your side.
5. Closing accounts. - I think we have all done this. You get a great offer in the mail "O%APR on transfers" I know I have. But one thing you should remember is not to close the account that you just transfered. Keep it open. If you close it, it could project your credit rating lower.
6. Not knowing. - Every person should check there own credit. Log onto MYFico.Com and view your rating as well as your history. If you are not checking your credit at least once a year then how would you know if your identity was or has been stolen?