I was having a discussion with my trainer about my lazy warmblood who waves a white flag at the mere sight of a hill. I was advised that horses need to be taught how to get up hills with power and energy and that like anything its just correct training.
My trainer said that with several of her horses she has taught them to really power up the hill in trot and canter through using other horses working alongside and/or a schooling whip to make them stay up into the bridle and not slack off. The outcome was at a hilly event the horses took one look at the hills and powered up really well.
I am probably as guilty of anyone of letting the horse stay into the bridle and trot up the hill but I do not ask for that extra 10% where they are really powerful and looking to take you up. I will definitely be working on it this winter.