Rider Position
I've found through my training over the many years, more often than not, if I'm not communicating with the horse effectively, 9 times out of 10, it is due to my seat.
Check this article about the importance of your seat as a driving aid. And remember.... always LOOK up!!!
http://forequestrians.com/Articles/Lynn-Palm/Rider-Position-Part-1/?C=6085