(MONEY Magazine) – Paying off your mortgage early can save you thousands of dollars in interest. For example, by prepaying $50 a month on a 30-year, $100,000 mortgage at 10%, you could avoid owing $60,322 in interest. One simple strategy to recoup that windfall is to add a little extra to each month's check to your lender. Most lenders permit prepayments if you get approval from them first. ''It's a painless way to save a small fortune, build up equity in your home faster and achieve early freedom from debt,'' says Marc Eisenson, author of A Banker's Secret: Your Mortgage Is a Great Investment (Good Advice Press, P.O. Box 78, Elizaville, N.Y. 12523; $11.95). Making those extra payments takes discipline, however, which could leave you prey to a new scheme being promoted by upward of a dozen firms across the country. Their salesmen -- sometimes financial planners -- contact homeowners and offer to make it easier for them to speed up their mortgages by turning their loans into what are called biweeklies. The deals work something like this: every two weeks, a bank hired by the firm withdraws from your checking or savings account an amount equal to half of your monthly mortgage payment. The agent bank then holds the money and makes your regular monthly payments to your lender, plus a 13th payment at year-end. This service does not come cheap: start-up fees run as high as $599.

The biggest of the bunch is a four-year-old outfit in Cape Coral, Fla. called CompuWorld International, which markets what it calls the Mortgage Reduction System. Only a few years ago, CompuWorld's founder, Russell Whitney, appeared on late-night, get-rich real estate TV shows crowing about how he could teach viewers to rake in fat profits by renovating dilapidated houses. Lately, he has been conducting $39-a-person seminars across the country recruiting salespeople for his biweekly mortgage service. At one talk in Alexandria, Va., Whitney urged the audience to enroll ''so you can get that Jaguar or two-carat diamond you've always wanted.'' If you hire a firm such as CompuWorld, this is what you may find: -- High costs. No matter how big your loan, CompuWorld agents charge $500 up front, then $65 a year -- $2.50 for each payment. For a 30-year loan, these fees would total about $1,870. If you set up your own prepayment plan and use the $1,870 to pay down, say, a 30-year, $75,000 loan at 10%, you'd save $16,563 in interest. -- Liberal use of your money. CompuWorld doesn't actually mail in payments every two weeks. It invests customers' money in Treasury bills or bank CDs and pockets the interest. At the end of the month, CompuWorld's banks send the mortgage payments to the lenders, who receive the 13th payment at the end of the year. -- Inexperienced staffers at relatively untested companies. Few of these firms have been around for more than five years, so there's some reason for concern about their staying power. For example, a year ago, United Western Funding of San Diego, a company that arranged biweeklies, filed for bankruptcy and ceased this service.