Never exceed the limit of your credit cards, or get close to it. If possible, use no more than 30% of your credit card balance. Your credit score will suffer if you are constantly near, or over the limit.
Don't use 5 different credit cards simultaneously and have them all charged with various amounts. If you can, put all those charges on a single credit card. One of the factors to determine your credit card balance is how many of your credit cards have balances. The fewer, the better.
If you successfully repaid an old debt, meaning that you paid it on time and under agreed terms, leave it on your credit report so that anybody who checks your credit report can see you are a responsible person who handles his/her debts well.
When you take a cash advance, that might indicate that you have money problems, or that you will have them in the near future. This could lower your credit rating, so try to avoid it if at all possible.
If you are not sure you'll be able to improve your credit score on your own, try using a credit repair company. Be sure to know how much will it cost you and what exactly are they able to do for you that you can't do on your own.
Often canceling old cards and opening new ones will look bad on your credit score. The longer you have an open account with the same credit card company, the more reliable you will look to potential money lenders, landlords, employers, etc.
Regularly check your credit report and complain about any mistakes it might hold. Everybody make mistakes, so don't take your report for granted. Study it carefully to see if everything written in it is true.