I had a friend who would leave notes on cars that were poorly parked. A typical note would go, "Nice parking job. Practice driving in straight," or "Horrible parking job. DOUCHE." The possibilities are endless.
I have thought about doing this, but who has the time to hand write a note and then put it on the car? So, I have decided that I will type out a few for future purposes. Contact me if you are interested in getting some.