How long can this go on?

Mortgage rates plunged to a new record low for the sixth time in the past seven weeks.

The average rate for a 30-year, fixed-rate mortgage -- the most popular type of loan -- fell to 4.71% in our latest survey of major lenders taken July 28.

That's the lowest average since Interest.com and its print predecessors began the survey in 1985.

Interest rates like we're seeing this summer make financing a home incredibly cheap. A once-in-a-lifetime cheap.

Just look at the best deals on our Mortgage News page.

Or search our extensive database for mortgage rates in your area.

In most cities, you'll find lenders offering 30-year, fixed-rate loans for 4.50% with no points and fees of $2,000 or less.

That means your principal and interest payments will be just $507 a month for every $100,000 you borrow. (Our mortgage calculator can show you the payment for any loan, any amount, rate and term.)

By any historical standard, a 30-year, fixed-rate loan that costs less than 6.5% is a good deal. So what does that make today's rates? A once-in-a-lifetime good deal.

Opt for a 15-year loan -- a popular alternative for refinancing -- and you'll be able to cut that to 3.875% with payments of $734 a month for every $100,000 you borrow.

The average cost of a 15-year mortgage was 4.17% in our most recent survey, also a record low.

Qualify for one of these loans, and you'll never have to refinance or worry about your mortgage again.

They're safe, totally predictable loans that carry none of the risks associated with interest-only or adjustable-rate mortgages. You'll never have to fret about interest rates going up, principal payments kicking in or any other nasty surprises that could drive up your housing costs a few years down the road.

Rates were expected to rise after the Federal Reserve ended its campaign to flood the mortgage market with money this spring.

But concerns that the economy is recovering more slowly than expected, and that several European nations might default on their debt, has sent investors fleeing for the safety of Treasury bills or other debt guaranteed by the U.S. government.

That includes mortgages, because 96% of all new home loans are now backed in some way by an agency of the federal government.

That surge in demand has actually pushed interest rates down since the Fed bought the last of the $1.25 trillion worth of home loans it now holds in late March.

You've probably heard that banks and mortgage companies have tightened their requirements for getting a mortgage after unwisely lowering their standards during the housing boom.

They have, and we've heard some real horror stories about all the hoops and delays borrowers have endured to get their loans -- or a big fat "no."

You'll have the best chance of getting approved, with least amount of hassle and the lowest possible rate, if you:

Have at least an average credit score. That's a FICO score of 720 to 730. Here's where to see what goes into your credit score and how to get your credit score.

If you have below-average credit, you should probably pursue an FHA or VA loan.

Borrowers with lower credit scores can increase the odds of having their application approved by getting the government to guarantee the loan's repayment.

Earn enough money to repay the loan. Here's where to learn about the two affordability tests most lenders use.

Can fully document your income, assets and debts. Here's where to find out about questions you'll be asked on a mortgage application and all the paperwork you'll need.

By Mike Sante

Interest.com Managing Editor