I have read where these A/F kit O2 sensors can go bad. What would cause one to go bad early?

If these are plugged in all of the time, how can we keep these working properly on our old cars that do not get used as daily drivers?

Thanks

Just a small amount of tetra-ethyl-lead gasoline additive will ruin the 'solid electrolyte' of either a narrow or wide band oxygen sensor by coating the surface.

The gasoline additive MMT will also do this,

http://en.wikipedia.org/wiki/Methylcyclopentadienyl_manganese_tricarbonyl

but slower than lead.
MMT can be legally used in 'no-lead' gasoline in Canada, but is used much less these days with ethanol blends.
MMT is in almost all the 'octane booster' that can be bought at auto parts stores.

The motor oil additive ZDDP

http://en.wikipedia.org/wiki/Zinc_dithiophosphate

will also slowly ruin an oxygen sensor if a particular cylinder is burning a good bit of lube oil for some reason. The EPA also believes ZDDP ruins the catalytic converter over time, but that is a bit controversial.

Some O2 sensors by design allow a tiny bit of air to diffuse into the sensor through the electrical wiring leads, and can go bad if excessive water comes in through this route:

http://en.wikipedia.org/wiki/Oxygen_sensor#Sensor_failures

ZDDP:
I contacted the manufactures of ZDDP with the question if their product would hurt an O2 sensor. This is their reply:

""Carburetors for the most part were eliminated in the 1980s. This means that up to 1996, many high-quality oils contained over 1600ppm of phosphorus in the form of ZDDP. These cars ran with high ZDDP and O2 sensors for years with little problem.""

Thanks for the reply. It was very informative.