I second B_and_D's suggestions.
Also, it should be noted that divorces have erupted from spouse/spouse interactions about backing up a trailer. So use caution.
The spotter should use specific detail as to the trailer/vehicle position. Say, "trailer needs to move 1 foot to the right", instead of "move to the right", the latter of which has very little value. Spacing your hands, and continually adjusting that spacing as the vehicle moves, to represent the space between the trailer or TV and some immovable object is VERY helpful.
The driver needs to realize the spotter is only trying to help the driver, nothing else. If it were not for the spotter, the driver would be significantly handicapped.
These need to be incorporated into regular marriage vows.
Due to our very tight driveway, my wife and I can now consistently park the TM within 1-3 inches of a concrete wall next to our house. It takes teamwork to do that.
Practice, practice, practice.
Dave