Like everyone else said, don't lean. Not because it's natural, but if you lean, your horse has to scramble to catch up with your weight. When you go through a turn (or in a circle) make sure the horse doesn't drop his shoulder (makes him feel like he's leaning to the inside) or pop his shoulder out (his head turns, but his body doesn't seem to want to follow!) Depending on how you're riding him, the next part is up to you. Western and HUS quarter horses and paints are taught to support themselves--when they go through a turn, they should hold their shoulders up without help from the rider. In more traditional english and dressage, the rider supports the horse through the turn. (To teach self carriage, you just support-release, then support again to show the horse where he should be, and release again and again and again... fun! Lol) To support your horse, make sure you're using leg and rein (and not just rein, because that will teach him to leave his shoulders everywhere you don't want them... lol).