People can usually estimate their credit score fairly accurately by taking an honest look at themselves. The FICO link above is also necessary.
35% Payment history. Is every account paid in full and on time, every time? If not, change your behavior and wait. If you pay on time and in full, companies should trust you more than someone who doesn't (remember the credit score is a measure of trust and nothing else). This part is usually well-understood.
15% Length of credit history. Only time can affect this. If you have no history or a short history, you'll be harmed. About the only thing you can do is to make certain that you use your old accounts on occasion. Having an old credit card means a company has trusted you for years. But it means a lot less if you haven't actually used it recently.
10% New credit. Don't go on an app-o-rama spree and you will probably ace this section. But also, being afraid to ever open an account can also hurt you. They like it if you show that you are still active and interested in credit (ie do you trust yourself?).
10% Types of credit used. Here is where people hurt themselves without knowing it. Is that USAA card your only card? If so, you have harmed your score here. The optimal score is to have a good mix - which means two or three credit cards in use. Also, to maximize it you need a mortgage, a retail account or two, and/or installment loans (auto or student for example). Avoiding debt just means that you don't trust yourself. Thus, your credit score should be lower since companies shouldn't trust you either.
30% Amounts owed. Here is the question that you asked about. The formulas are changing, but you are probably good if you use less than 30% of your available credit. That is, you have lots of cushion room left if you get into bad times. But this also applies to other loans. Is your car loan mostly paid or mostly unpaid? Same goes with the mortgage. You need to show that you have at least one credit card with $0 balance. But you shouldn't have all with $0 balance. Note: you can pay it off in full and still not have a $0 balance if you charge something else the next cycle.