Are you talking like a horsemanship or equitation class?? Everytime I ride I try to sit the way you are suppose to in a class, it just helps build up the muscles needed to do that. For western, Sit up stright, legs underneath me, heels down, shoulders back, head up... it should almost be like if the horse was removed underneath you, you could stand (if that makes sense). Horsemanship classes are judged heavily on how you sit, and how much movement you have while on the horse. Some shows also call out patterns in which you are judged on how well you do the pattern and how well you ride...
hope this helps
It's not the will to win, but the will to prepare to win that makes the difference.
- Paul "Bear" Bryant (Former college football coach)