When they are just everyday driving, tell them to use the side mirrors on their cars. Learn what the side mirrors are showing you. When they back up, tell them to practice NOT looking over their shoulder.
When they have a spotter helping them to back up, have the spotter tell them which direction they want the rear of the trailer to go, try not to tell the driver which way to turn the wheels.
Obey the speed limits, especially on side roads and roads they don't know.
There are thousands of tips for towing... as others said, nothing beats practice.