Source: myFICO.com; rates as of May 7, 2007
Be too thin
When it comes to your credit record, fat is good, emaciated bad.
Even if you're the most responsible, on-time, in-full bill payer on the planet, your credit score won't be as high as it could be if you have just one credit account.
The reason: Your credit profile is too thin and lenders ideally like to see a potential borrower responsibly managing a mix of revolving debt (such as credit cards, where you can reuse the credit after paying it back) and installment debt (such as a car loan or most mortgages, where you pay the same amount every month for a certain period).
To boost your score: Consider opening another credit-card account or two, or taking out a car loan or small bank loan.