The best tip I ever received on how to back a trailer I still use today. Put your hand on the bottom of the steering wheel, look over your shoulder at the trailer and as you back, move your hand it the direction you want the trailer to go.
I struggle, however, if I have to back a friends trailer and can't see the trailer because of the tailgate or 3rd row seat in an SUV. Some of my friends back only using their mirrors, but that takes getting use to.