+1 on this idea, and go twice as slow as you think you should.
I have also laid a 6' rod across the back of the trailer so I could see the ends in the truck mirrors.
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This technique is a good one to learn...backing up looking in the mirror. With practice, it is fairly easy to do. Just needing to look back for any obstructions, like buildings that need to be missed or aligned with. That right hand mirror (objects are closer than you think) is a pain.
I don't "memorize" righty/lefty motion on the steering wheel. Just look and go, as it is soon apparent which way is the wrong way.

But whatever works. I think practice makes backing up intuitive, and not mechanical.