The three things that have the biggest impact on your credit are Credit History, Available Credit, and Years of Credit.
1) Credit History: Represents 35% of your Credit Score, and it is the history of how well you have paid your bills during the time you have had credit. However, late payments, charge offs, or collections that are over two years have very little impact on your current credit score. In fact it is better to not pay off a collection that is over two years old, because if you do you will bring the last reporting date current again which will be a recent late payment.
2) Available Credit: Represents 30% of your Credit Score. Available Credit is just what it says, it is the amount of credit that you have available to you. For example the number of credit cards you have, and how much of the credit limit on the cards are still available to you. If you are over 30% of the available credit on a credit card it has a negative impact on your credit. So a quick way to improve your credit is pay down your credit cards to less that 30%. This has a huge quick improvement on your scores.
3) Years of Credit: Other wise know as length of credit, and it represents 12% of your Credit Score. This simply is the number of years that you have had credit, the long the number of years with good credit the higher the score.
Also one other point closing out unused credit cards does not help your credit. You are better off keeping them open, because it adds to your available credit.
I hope this helps.