Measuring battery voltage and saving energy
I have done this before, I used a PFET to turn off the current through a 10k-10k voltage divider. The ADC on an Atmega should not have more then ~10k of impedance on the input or you need a capacitor or something to help buffer the amount of charge available to be read in by the ADC.
Basically the PFET was always off, and only upon waking up was the voltage divider turned on for a short period of time.