If you tend to grip with your legs, and lean forward when you canter your dressage horse, here's a simple exercise to help you sit up straight.
Sit in a chair and do the following exercise for muscle memory:
1. Sit up straight with shoulders over hips.
2. Lean back about 4-5 inches
3. Come back up to the vertical.
4. Do this in the rhythm/tempo of the canter.
5. Notice how as your upper body comes back, your hip angle opens.
6. Close your eyes as you do it, and memorize this feeling of the hips opening and the upper body rocking backwards.
Now try these "backward rocks" for real on your horse, but mimic the feeling you had through your body when you were sitting on your chair.
A Happy Horse