Dr. Ulf Moller, works with a 6 year old stallion, achieving the bend, flying changes, tension in changes, and straightness.
Ulf emphasizes the necessity of always remembering to use your corners for bend. Out of the bend, you must remember to achieve ultimate straightness. Think.....bend and straight lines, always. Riders often first think to pull back or increase contact on the outside rein to straighten, however, if you have maintained correct contact, your should instead think of slightly pushing your inside hand forward to straighten. He has Eva alternate the sitting trot with the rising trot, ensuring the horse understands when she sits, he gives a little more collection and attention. Once you go back to rising, it is like saying, Yes, thank you, here is your reward. Then move into lateral work, shoulder in/haunches in and taking that into the half pass.