I continued to work at it last summer until I could more or less sit the medium trot without too much air beneath my butt. Chemaine promised me that as Speedy's medium trot got more balanced, more powerful, and more uphill, the medium and extended trot would be easier to sit. She was right. I still don't sit it as effortlessly as I would like, but I am sitting it, and more importantly, I am actually creating the medium trot with my seat.
This series of screen shots is from one medium/extended trot across the long diagonal. Our extended trot looks suspiciously like our medium trot. Maybe the judge won't notice.