Create an amortization schedule for yourself. When I did this, I could see how much was paid in interest each month, and how much in principal. At the very beginning of your mortgage, the principal payments are relatively small. I would add in at least one extra principal payment, essentially knocking out 2 months. Towards the end, much more is going to principal, so doubling it is harder. Thus I focused my prepayment efforts on the beginning. I could easily add 3 or 4 months worth of principal at a time, or sporadically add larger lump sums to knock out years at a time. The last year we simply paid as scheduled, but we paid our 30 mortgage off in less than 15 years, despite low income, 5 kids, SAHM and DH unemployed for a year and going to grad school somewhere in the middle.