Before I had a backup camera I used to carry one of those orange highway cones. I would get out and put it exactly on the drivers side corner where I wanted to stop. Then I would line up and back with my left mirror until my bumper tilted the cone back. I stopped, got out, put the cone away, done. Best spotter I ever had.
Then one day I was backing into the driveway and my wife was in my mirror waving that hand mumback, mumback fast motion then she raised her hand to stop just as I hit the garage roof with the ladder. First thing I thought was where are the hand signals she knows to use. Second thing I thought was how much I missed my cone.