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.

good idea
If you don't have a camera, ask a rental office employee to walk around the
vehicle with you, list existing damage, and sign the list. The signature of an
employee may be better than a photograph.
Thanks, Joe, you are absolutely right. I was suggesting this in addition to checking the car out with the employee, just in case there becomes an issue of "no, that scratch is bigger than it was when you picked up the car." I should have explained that better in the post! I'm glad you cleared that up.