If you're experiencing this problem, and particularly if the key won't turn from position 0, or is becoming increasingly difficult to turn through any of the positions, try removing the locking assembly from the car, and then flooding the lock cylinder through the keyhole while in the keyhole-up position. (There are plenty of good posts about how to remove the assembly from the car) Then give the oil a couple days to disperse inside all of the tumblers, etc. before giving up on this approach process.

I would particularly recommend making a slurry of motor oil and powdered graphite lock lubricant (HomeDepot), and flooding with that in order to keep the cylinder operating easily for longer.

I did this a few days ago, and the results were excellent!