I use a dandy brush so it doesn't pull as much hair out xx
I brush my horses mane with a stiff brush to help stimulate the oils so that it's not dry, and also it won't pull out as many hairs as a normal mane comb would.
Pretty much what Alexia said! Wash, condition, let dry completely and then braid. The braiding protects it from breaking and you need to take it down at least once a month to wash, condition, braid all over again. I also use Healthy Hair Care spray, daily on the whole coat, mane and tail.
It's mostly genetics for the most part on a really long mane. The rest is keeping it clean so it's not itchy and the horse rubs it out. Wash it, rinse really well, condition it with something that has no drying agents, get the tangles out with your fingers, let it dry and then keep it braided. Brushing it pull hairs out. Also a big part is just making sure the horse has good nutrition in general :)