According to info Crossroads Bob posted in the past, the internal clamping diodes
in the ATmega chips can handle at least 5mA. Therefore, another possibility might
be to use just a 4.7K series-R in the ADC lines. This would protect to over 25V,
and shouldn't affect the ADC readings since the datasheet says
A rubber zener will protect against overvoltage, but unlike a real zener will not protect against reverse voltage, and it will not react immediately to fast transients because of the collector-base capacitance and the switching speed of the transistor.
Unless you are measuring signals that change rapidly, a series resistor is all you need. For example, 22K will protect against about +/- 40V. The important thing is to limit the current into the internal pin protection diode to 2mA or less (I normally design for a few hundred microamps at most).
[EDIT: oric_dan got there first. Not sure where the figure of 5mA came from, I've seen 2mA in an Atmel application note.]