I'm a novice rider, just started in September and have been riding once a week. So far, I'm perfecting my posting trot and will go over trot poles or a cross-rail every once and a while. But it seems that every time my instructor tells me to trot, she'll ask me again a few seconds later to switch my posting. I sit, and go on the right lead. Every time I try to predict this and sit thinking I'm on the right lead, she still asks me to switch. So, what is the rule that you follow for posting? Do I rise up as my horse's inside shoulder goes forward? Any help would be appreciated, thanks!