There's a fairly simple way to reduce the ability of Big Brother to track you using your metrocard. Purchase it with cash from a vending machine, and do your best to avoid looking at any security cameras nearby. If you were really paranoid, you could always buy three or four and randomly select which one to use whenever you get on the train...I guarantee, trying to correlate use patterns would be completely pointless given the data. Add a different-length delay between use each time (and if you're commuting, even a minute or so helps) and you're totally anonymous.

Now it goes without saying that you shouldn't let Them have your Metrocard or even be able to grab and examine it! When it's expired, destroy it; if you think anyone's managed to read it, destroy it; if you're feeling particularly paranoid that day, destroy it.

Being a blank is possible in the age of surveillance. It just takes some thought and effort. Jam the system!