A good way to start this is by doing 'carrot stretches' from the ground.
- Getting the horse to reach from the left/right to it's flank, as far as it can reach. The further the distance the more flexion.
- In between the front legs up to its belly, almost at a bow.
- Your hand up in the air and your horse must reach up to it's highest point to stretch those muscles.
That's a good place to start, if a horse isn't flexible from the ground, it won't be being ridden.
When a horse is working in a true outline, it's the entirety of its body meaning you need the high quarters to be working just as hard as the rest of it. The horse needs to track up and be encouraged into the bridle. Lunging in a pessoa is a good way for you to start that. The training aid encourages the horse to step through and work its topline.
When ridden the needs to be focused and not bored otherwise will become flat. The paces need elevation which in turn aids toward a correct outline. Plenty of transitions will help with this (halt to trot, canter to halt and so on) and lots of shapes. You need to be strong in your seat and be giving him your aids through your entire body, not just leg and hands. Every part of you needs to work independently. Your hands need to be carried higher than usual, not to carry the horse but so your arms/hands are separate from your body.