FICO scores are calculated based on the following, which are listed in order of importance:
35%: Payment history. This one is simple. Just pay your loans and credit card bills on time.
30%: Amounts owed. This has to do with something called your credit utilization ratio. Essentially, you don't want to be using a high percentage of your total available credit. For instance, instead of maxing out your credit card, aim to spend no more than 30% of your credit limit.
15%: Length of credit history. Generally, the longer you've been using credit, the better.
10%: Credit mix. It's best to have a variety of accounts, including revolving debt (like credit cards) and installment loans (like mortgages).
10%: New credit. Refrain from going overboard and applying for a bunch of new loans and credit cards in a short period of time. It looks like you're desperate for credit.