I own a 1997 Infiniti QX4 with about 108,000 miles.  The Service
engine lite came on, I had it read and it was o2 oxygen sensors.  They
replaced them and all was fine for about two weeks, then the lite was
on again.  Back to the mechanic and he replaced them again, used a gas
additive to clean the system and all was well for about two months. 
Now the lite is back on so I go to another mechanic that reads the
codes, and guess what - O2 Oxygen sensors.

Otherwise the car is running fine with no degrading in the gas mileage.

What do I do?

My car has the same issue.  I had the O2 sensors replaced and the gas
additive, and no luck.  I'm ignoring the problem and having no
problems other than the light constantly being on (which could prevent
me from seeing another problem in time to nip it in the bud).
My best guess is that it is a computer error and replacing a computer
in a car with over 100k miles to get the check engine light off just
isn't worth it.