You can do a lot a pre-payment experiments yourself online, with calculators such as http://www.bankrate.com/brm/mortgage-calculator.asp. (I used $100K, 6%, 30yr fixed).

So, how much does paying half of each payment half a month early speed things up? Let’s simplify: make a full extra payment at the start; this is the same as paying all of each payment a full month early. How much time does this take off the loan? About 5 months — not bad for a single payment, but not a multi-year miracle, either.

Adding 10% extra to each payment shaves off about 6 years; 20% shaves off 10 years. If you’re looking to pay off early, such a habit is the way to go. Keep it simple, don’t look for miracles in software or complicated schemes.

(Just keeping track of twice the number of payments over 30 years would drive me nuts, and would make the 5-month savings not worth it to me!)