J$ has posted about paying off his mortgage with a plan. He used a calculator for looking at how much he can save interest by putting extra month every month. I am much more of a spreadsheet addict. I track my mortgage payment with the mortgage amortization spreadsheet. This way, I can look at how much extra i put in and when.
At the top is your basic numbers from the loan. However, some the spreadsheet is protected. I would unlock it and put in extra cells, one for the amount of interest I would pay if I don't pay extra principle, and one for the interest I am saving with early payments. I do this with my investment mortgages as well. When I refinance a loan, I would copy the old spreadsheet and change the numbers in the new one, so I have a record of what interest I paid already on one house. Seeing the numbers is a encouraging way for me to put in the extra payment.