This article by Practice Horseman Magazine is semi-relevant to dressage (it is oriented mostly toward jumpers).
It addresses ..
- Gripping with your knees.
- Hunching shoulders and collapsing your torso.
- Too much hand in turns (causing loss of shoulder).
- Bouncing during sitting trot.
- Losing the stirrups.
Link to article: 10 Time-Tested Rider Position Fixes