Before starting your trip take check-in photos of your car condition.
This is the one reason why I am using rentalmoose.com because it has an easy to use car documentation feature that reminds me to take always pictures of the outside and inside condition.
This way if I have any arguing with the rental car company I have proof of the car condition.
Here is a detailed guide on how to make quality pictures of your car.
After that you are ready to start your trip.