In my upcoming lessons (after I gain better control of the horse) I am going to ask the instructor to specifically work on posting while looking at my position.
On Sunday I was trying to use my feet and legs in the stirrups to force myself to go up and down, which is not the advice here. I was definitely not using the motion of the horse to help me and it sounds like that is what I need to do. Wow, lots of work and practice ahead! It was near impossble for me to go fast enough to keep up with the horses motion, trying to do it myself and not using the horse's motion to assist me.
When the horse is trotting and you're bouncing up and down, at first I thought I was posting just because my butt was coming off the seat without having to do anything but sit there. I know differently now!