Want to improve your credit score? Concentrate on the areas that matter most to lenders.

1. Pay your bills on time. More than one-third of your credit score -- the most popular being the FICO score (from Fair Isaac and Co.) -- is based on your bill-paying habits. Late payments can have brutal consequences, particularly any recent tardy slips. So don't skip any bills (certainly not your rent or your mortgage payment) -- send in just the minimum amount due, but send something. If you know your payment will be late, call your lender and explain, and he might give you a free pass just this once.

2. Don't max out your credit cards. You might have a $15,000 credit limit on your card, but that doesn't mean that's how much you can afford to spend. Keep your debts to 35% or less of your credit limits. Red flags start waving when your debt-to-available-credit ratio exceeds 50%. Living within your means counts for a whopping 30% of your score.

3. Create a credit history. The longer your borrowing history -- particularly if you've been a responsible card-carrying citizen -- the better your score. The length of time you've spent in the system determines 15% of your overall score. If you're going to close some accounts, it might make sense to cancel newer ones since the old ones help establish your long and illustrious credit record. (Here are a few pointers on picking cards to cancel.)

4. Avoid frequent card-hopping. If you loaned someone money, you'd probably get nervous if they started asking all their other friends for a loaner, too. Same with professional lenders. Applying for lots of credit makes them tighten their purse strings and fire a few warning shots at your credit score. Still, new credit applications have just a 10% impact on your score. So if you're trying to pay off debts, shopping around for the best deal makes sense. (Fools call this "snowballing.") But remember, every line of credit you apply for will stay on your record at least seven years, even if the account is open only for a day or two. So take great care when opening and closing accounts.

5. Don't borrow money just to boost your score. Those with the highest scores have a proven history with different kinds of credit -- such as installment loans (like a car loan or mortgage) and revolving debt (your workaday credit card). But paying interest -- or annual fees or other costs of borrowing -- just to boost your score isn't worth it. If your score is 750 or higher (on a range from 300 to 850), you probably already qualify for the best rates. This measure counts for just 10% of your overall score. Remember, time and responsible bill-paying habits matter the most.

6. Know who's checking. These days your credit score can affect a lot more than just your interest rates. Insurers check it to set your car and home insurance premiums. Potential employers use it to help them size you up. A lot of folks have the keys to your credit file. Each time a potential lender or insurer checks, a "hard inquiry" is recorded in your file. (When you or an employer accesses the file, it's a "soft inquiry" and has no effect.) Occasionally check your credit report to make sure those who are being nosy have a right to be.