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.
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.
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.
Talk to your lenders if you can't make payments on time
Being late on your payments, or missing them entirely, will decrease your credit score. If you know you won't be able to honor current payment schedule, talk to your lenders to make a new payment plan, the one you will be able to sustain. That way you will avoid damage to your credit score.
If you order supplies from other businesses, try to establish a trade credit with those suppliers. That means you will be able to pay days or weeks after you received supplies. If you honor your agreement and pay on time, your credit score will increase. You need at least 3 trade lines for this to have effect.