I am a huge believer in riding horses in “long and low.” I think it keeps them happy and sound. It’s not an easy exercise for the rider, nor is it something that a horse who’s previously been improperly ridden will pick up in a day. But if you want a dressage horse to last long into his twenties, you oughta try it.
Here’s a pretty good video discussing long and low:
How to engage your horse’s back: