What I like best is starting by walking them in a right hand circle and making the circle smaller and smaller, starting to invade their space so they learn to move away from you as you walk into them. Eventually their circle gets very tight and they willingly plant their hoof and begin a pivot. As soon as they do that I back off and release all pressure and give them a pat. It's important that their front legs are crossing over in front and not behind.
Once they get good at that I will ask from a square halt, pulling them forward slightly as I walk into them, to encourage the crossing over, then slight backing pressure to encourage them to stay on the hind end.
For horses who aren't great about moving out of your space when you walk into them I will give them just one quick slap on the shoulder to make them more aware of my presence and tell them that they need to move now.