This will only be effective if you know that it's going to take you a while (at least a month) to pay off the mortgage, either because it's a big one or you simply like to take your time.

Instead of putting money straight into paying off a bit your mortgage, put the amount to be payed into your savings account, and every time you want to use money to pay off part of your mortgage, put it into the savings account instead. Then, once you have enough to pay off your mortgage, take the required amount of money out of the account and pay it off.

This should actually shorten the amount of time it takes to pay off the mortgage because you're getting a bit of money added to the account at the end of every month (more money in the account means more is added on).