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NEW YORK (CNN/Money) - Your home is your biggest and most important investment. To maximize its value, you need to find the best mortgage.

However, mortgage lenders just revealed in recent reports that not everyone gets the same interest rate. The difference can mean affording or not affording your dream home.

How can you make sure you get the best interest rate? Here are today's five tips.

The single most important element to your mortgage application is your credit history. Having a bad credit history penalizes you. It says to lenders that you can't pay your bills on time and gives them reason to charge you a higher interest rate.

On the other hand, many households don't have enough of a credit history to apply for a mortgage, according to Evan Hendricks, a credit counselor and editor of Privacy Times.

If you always pay in cash or have only one or two store credit cards (i.e. a Sears credit card), you might have a so-called "thin file." Lenders might see you as a risk, because you haven't established enough credit relationships with other lenders to prove you are a worthy borrower who can pay bills on time.

If you suspect you have a thin file, you need to fatten it before you shop for a mortgage. Hendricks says you can establish yourself in the credit world by getting one or two major credit cards, say a Visa or MasterCard.

Use the cards a little each month and be sure to pay your bills on time. Within six months, Hendricks says you can build a good credit score that will help in applying for a mortgage.

When it comes time to start shopping, get a copy of your credit report to know exactly what the lenders will see.

If you live in the West and Midwest, you can get one free copy annually from each of the three major credit bureaus: Experian, TransUnion, and Equifax. Residents in the South will get that right starting June 1st and those on the East Coast will also by September 1st.

You are given a credit score based on your file. Unfortunately, you have to pay to find out what your score is. Hendricks suggests you get yours from either myFICO or Equifax, who are more reliable. The more information you have on your credit history, the less the lender can take advantage of you.

If you find your credit history is a mess because of mistakes or fraud on your credit report, you will want to fix them to ensure you get the interest rate you deserve.

The process can be daunting. But here's a little-known secret Hendricks told us: you can get some help cleaning up your file, usually at the expense of the lender.

To take advantage of this opportunity, make sure any broker you are considering can recommend a reseller. Resellers are in charge of what is called rapid re-scoring -- the process of reviewing your credit reports, finding errors, and helping you fix the problem to get a new credit score quickly. Watch out for re-scoring scams by only using a reseller recommended by your broker.

Of course that also means you need to find a legitimate mortgage broker to begin with. Your bank or credit union can offer mortgage services.

For smaller mortgage companies, check their reputation with the Better Business Bureau or see if they are certified by the National Association of Mortgage Brokers' Consumers, which requires lenders follow a certain code of ethics.

Remember, you have the right to fight for the best deal on your mortgage. Know your credit score when you go in to talk to a lender, but keep in mind it's only a ballpark figure.

"Consumers need to know there are multiple scores out there. Loan officers are going to throw out the highest and lowest and take the middle," says Terry Clemans, executive director at the National Credit Reporting Association.

In fact, the average consumer has a 40-point difference in their credit scores. But it is still worth buying yours to get an idea how much interest you will have to pay on a loan. Check out MyFico.com's Loan Savings Calculator. It can help you in negotiating with the lender.

That's right, negotiate. Don't be afraid to ask questions: If the rate is higher than your ballpark figure, ask why. If you can see your credit history needs improving, ask "How much lower can my interest rate go if I take six months to fix my credit?" The more engaged you are in the process, the better deal you'll make.

Getting your best interest rate is important. Slight differences in your rate can amount to quite a lot of money over the life of your loan.

Chris Kemper, spokesman at National City, one of the nation's largest home lenders gives an example, "Just a half percent rate difference on a $100,000 loan at today's rates can mean a difference of over $30 each month in interest. That's $360 each year, or more than $10,000 over the life of a 30-year loan."

But remember, your interest rate is only a part of your mortgage's overall package. You also need to watch out for high fees and make sure you can meet your monthly payments.

If you are still overwhelmed, the Department of Housing and Urban Development offers a list of approved counseling agencies that can be great resources on its Web site.

Gerri Willis is a personal finance editor for CNN Business News and the host for Open House. E-mail comments to 5tips@cnn.com.