What my aunt does everytime before mounting her gelding, is circle him around her. Getting his hindquarters engaged and moving away from her. He used to be real bad about moving when she was mounting too. She circles him to the left and right a few times. If he still moves when she mounts she backs him up a whole lot. I do the same thing with him. I get pretty rough with him when he does that, just because there is no reason for it, especially when we are using the mounting block. When I say rough, I mean, I back him up as fast as I can and for as long as I want, and even circle him in real tight circles left and right. Than I'll dismount and try it all again, first circling him on the ground left and right, try mounting (constantly telling him to 'stand'), than so on and so on. He's doing much better now!