This might be old news for you younger, smarter folks, but I learned an amazing little trick today. When picking up a rental car, go around and take pictures of any damage that is on the car when you pick it up. If your phone or camera has a date/time stamp, use it.
These pictures will come in handy if the rental car company notes damage when you return the car. Yes, those pictures might not hold up in a court of law, but most rental car companies will just give up on pursuing damages if you show them pictures.
Another layer of protection never hurts.