You have already proven that you can live ten months out of the year on two (or four) paychecks, so save the extra income received the other two months. In order to save the extra paycheck you need to split the savings up over the next three paychecks so you can continue to pay all of your bills and save the paycheck. The extra paycheck comes at the end of the month, right before bills are due, and it is easy to miss this opportunity if you are not prepared to take advantage of it.
For 2013, March, May, August and November have five Fridays, so the last Friday of these months will be the extra paychecks we will target to save. Once again rent/mortgage payments are generally due at the beginning of the month so saving all of the third paycheck is nearly impossible, unless you have a plan.