Amounts owed. Having a lot of credit is great, but using it is also important. However, you can't use too much or your score will be damaged. Ideally, you want to utilize no more than 30% of your credit at any given time. The amount you owe contributes 30% to your score.
Length of credit history. How long you've had credit matters, but so does how long you've had any specific account or how long it's been since you've used your accounts. However, this is only 15% of your score, so don't panic if you're new to credit.
Credit mix. Having a mix of credit types is also pretty important. You want to have a revolving credit line, like a credit card, a mortgage, and other types of credit, for the best results. This only counts for 10% of your score, though.
New credit. You've probably heard that it's important not to have too many inquiries, and that's partially what this metric is about. It's also about not having too many new accounts at once. Instead, spread out new account applications by a year or more, if possible.