I mentioned this in another thread, but I think it's worth repeating.
I have trouble with hand eye coordination and therefore had a horrible time learning how to post. With a trainer yelling at me, it was frustrating, to say the least!
But I learned a visualization technique that really helped. In a nutshell, here's what I did: I closed my eyes and pictured my horse trotting. Then, with my eyes still closed, I moved my arm back and forth to simulate the motion. As I did that, I said "up, down, up down" to myself and pictured myself posting on the correct diagonal.
It worked -- and once I learned how to post on the correct diagonal, posting itself became easier, too.
Hope this helps.