A day or two before the car is shipped, give it a good washing so that you can clearly see any damage. Document any existing scratches, dings, and rust spots (a digital camera may be helpful for this) so that you have a record of the condition in which you delivered the car to the shipper.
Empty the car of personal belongings. These may not be insured if lost in transit, and may become loose and damage the interior. Some shippers will allow you to transport a few lightweight objects in the trunk. Also, remove any third-party equipment like fog lights or spoilers. Most shippers ask that you leave the gas tank no more than one-quarter full. And be sure to deactivate the car alarm!