Perhaps you need to develop a move out cue. Your horse will learn to just stand there until you give a specific cue to move out. For my horses, I sit up in the saddle, and give a soft cluck sound. Until that cue sequence, I expect them to just stand there while I get myself and my stuff all adjusted. Of course retraining one that already walks off is harder. I would think the backing would work but you need some clear signal that says "OK to go". I ride a hot breed and I have found that I do need to work at the stand still until go lesson pretty frequently because they love to just take off if they can get away with it. You have to be consistent about it too. Just a time or two of letting them take off and you are back to having to train it all over.