When credit bureaus determine an individuals credit score they look at 5 factors.
Below are the 5 factors and approximately the weight each factor carries.

1. Payment History (40%) - Are your bills paid? Are they late?
2. Amount in Debt (30%) - This is your ratio of debt to available credit.
   Can you afford to make the purchase?
3. Pursuit of New Credit (10%) - How often are you opening new accounts? If it looks like you are frequently opening new accounts this may cause your score to go down.
   
4. Credit Experience (10%) - What types of credit do you have? Credit Cards, Mortgage, Car Loan, ...
   The more diverse the better.
   
5. Length of Credit (10%) - How long have you been using credit? The longer the better.
   

Credit bureaus don't want people to know how their credit score is determined.
This is because they don't want individuals to be able to manipulate their score.
The only factors an individual should worry about are payment history, and amount in debt.
If these two factors are good then your credit score will be high and you will pay prime (low) interest.
4 Tips to Improve Your Credit Score

1. Pay you Bills on Time - Delinquent Accounts and Charge offs will severely negatively impact your credit score.
   
2. Keep Low Balances on Unsecured Credit Cards - This will help your ratio of debt to available credit.
   
3. Information on Credit Report is Accurate - Dispute inaccurate information.
   The FCRA (fair credit reporting act) states that credit bureaus must delete any unverifiable or inaccurate listing.
   
4. Remove Negative Listings - Any negative listing will cause your credit score to be negatively impacted and force you to pay sub prime (high) interest.