You should always be leaving your car in gear when parked, always! Pull the brake first, so that the brake is holding the weight of the car, and not the transmission, but put it into gear. If the brake fails (the cables stretch over time, and most people don't bother to adjust them, so you have to pull harder and harder on the brake to set it properly), then you have the transmission to hold the car.
In case my point wasn't clear, never leave your car in neutral while parked.
I'm going to sound like an old man, but what do they teach to new drivers these days? I understand driver's ed isn't required, but so many basics (and not just etiquette, but actual laws) are completely foreign to most drivers.