> I usually place only a small cap and a zener

This is effective for Vcc-type noise but for some types of
noise a Schottky diode is essential. In two recent projects,
motors and relays caused a negative pulse on comparator
and A/D input pins that would have destroyed them. If the
pulse goes below 0V by 0.6V the PIC's internal diode conducts
and the pin is history. The Schottky limits this pulse to less
than 0.6V and prevents the PIC's diode conducting. A small
series resistor can be added too, eg 470R. Suitable diodes
would be BAT46 or 1N5817

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