It is impossible to tell you which criteria have the most effect on your score. However, the following points are a good place to start.
Any time you go past the due date on a loan, it is reported to the credit bureau and can lower your score. One day past the due date can get you a negative mark.
This can range from bad checks to medical bills. These are usually placed by collection agencies after the primary debt collector has tried to arrange payments with you.
The notation that you filed bankruptcy will show on your credit report for 10 years. Also, anyone you filled against has the right to report the amount of the loss they sustained from your filing. You can not ask for these to be removed, only for them to be marked that they were included in the bankruptcy.
Unsecured debt means that a person is paying higher interest. This is an indication that they may be living beyond their means.
High limits on your credit cards can hurt your credit, even if you don't owe anything against it. This is because the opportunity for you to run the cards up exists.
Using companies such as American General will adversely effect your score. Be careful about doing a lot of 90 days same as cash offers, since they normally use high-risk finance companies.
Have you gone shopping for a car recently? Then you should know that a car company will run your name through a number of finance companies while you go for a test drive. Thanks to recent changes in the scoring, these no longer count individually. However, apply for a number of different credit cards at the same time, and you will see your score fall. It's important to leave time between loan applications.
You can't prove your ability to pay if you don't have any loans.
If you credit card balance is close to or over your limit, this will pull your score down. You need to leave some room between these two numbers.
If someone has opened a number of accounts recently, it will appear that they are out of control on their spending.