It'd be a pain in the ass for zipcar, but every parking space should have an attendant to do a quick once over every time someone parks a car.

Less hassle with verifying who actually trashed it, but also if you have to stare someone in the face after your ride, you'll probably be more respectful of the car.

But really, there's just not a satisfactory solution I can think of to make me want to use the service.