For the first 5-7 years of a long term mortgage all of the payments made go directly to interest and not to the principal. This is why for the first few years the principal never seems to decrease very much at all.

Most homeowners will end up paying off their mortgage by the time that they retire, unless they can religiously make extra payments. If you want to make double payments on your mortgage, then you need to clarify with your lender every time you make the payment.

Thus, when you make an extra payment you must note that the money should go to the principal amount immediately. Otherwise, the lender may just think you are paying a month's worth of your mortgage ahead of time.