I basically question what a boss horse wouldn't tolerate, and that's what I don't tolerate. So I don't tolerate horses walking in front when I lead, I don't tolerate them stepping into my space without permission, I don't tolerate them nibbling (what a lot of people call "kissing"), no turning and walking away when I'm working with them, no head-shoving me around, no crowding at the gate when I'm coming in, and no turning their butt towards me when I'm free-lunging. The key and hard part is knowing when they're doing it or about to do it because timing is everything. If you let a horse do something and then wait 3 seconds before discipline, it's not going to have much of an affect. Or if you only discipline sometimes, it's not going to stick.