1 Too many credit cards, esp those with a very high credit line.
2 Too many credit inquiries in a short period. Each time you apply for a credit card, bank account, loan, cell phone, they will check your credit and leave a record.
3 Too much balance on your credit cards.

To get a credit report of yourself:
1 Join some association
2 If you apply for a credit card or a loan and are rejected, they will send you a letter and tell you to call a number (usually a 800 number) within 60 days to get a free report.