Great advise so far
Your extra payments are suppose to go directly to the principal of the loan; which is effectively like making once extra monthly payment per year. This does pay down the loan quicker, however it is less effective than simply reducing your amortization period.
You should not have a mortgage loan greater than 20 years and strive for 15 if you can handle the monthly payments? Anything longer greatly increases your overall interest paid.
Here's a mortgage calculator that you can play around with and test the optimal amount for yourself; but take it from me, go for the 15 year amortization and join the ranks of the debt free!
Calculator:
http://www.kentredekop.com/3276_mortgage_calculator.jsp
The calculator is on my companies website, which I AM an owner of, fyi.