You post with the outside leg as it goes forward, then sit back when his hoof hits the ground and moves back.
So when you go forward, kind of stand up in your stirrups then sit back down. Then just repeat. You just stand up - going forward not up - to where your hips are level with your knees would be the farthest you would want to go.
Correct me if I am wrong - that is the way I was taught.