Pay your bills on time. This action has the most effect on your credit score.
Pay down those high credit card balances. Do not charge up to the maximum allowed. It is true that the closer you charge to your credit limits, the lower your credit score will be.
Do not apply for loans or credit cards that you really do not need. It may look like you are taking on more debt than you can financially handle if you have many credit cards and outstanding loans. This can also lower your credit score.
Close those department store credit cards that you no longer use. Pay off any loans that have a low balance. Remember, it typically takes a full thirty days from the date you make payment, close an account, or pay off an account for that information to show up on your credit report.