The vast majority of failed O2 sensors I have seen here are due to being blacked out, much like a spark plug gets blacked out. If you run them very rich for a decent period of time, they will become very sooty appearing and this tends to reduce their life expectency dramatically. If everything is "by the book", the O2 sensor should be a gray or a tan color after it is run.
Like Butch said, you probably shouldn't clean the sensor with any type of solvent. It's not really a good idea to expose the sensor to a bunch of chemicals it wasn't designed to be exposed to. I've talked to many people who have tried this on a sensor that appeared to be dead and it has never once worked to my knowledge.
