[aephi alum]

Most lenders will apply any excess payment to interest first, unless you explicitly instruct them otherwise. If you make 2 months' payment at once, and don't instruct otherwise, they just won't send you your next bill; if you owe $200 a month and you send $400 in January, they won't send you a February bill on the assumption that you're prepaying your February bill rather than trying to put an extra $200 toward your principal.

My mortgage bills contain a space where you can write in an additional amount for principal and for escrow (they escrow for property taxes). We always throw a little bit extra toward principal and nothing toward escrow. On one occasion, I misread the bill and paid more than I had to, and the excess was applied to principal. (I love my bank.)

Oh... if you have several loans and you have extra money to apply to principal, apply it to your highest interest loan first. That means credit card debt, if you have any. By paying off your highest interest loans first, you'll be throwing away less money on interest over time.