To straighten the horse in walk, ride the quarterlines in shoulder fore position. Guard against the horse swinging his haunches right by almost always riding a bit shoulder fore.
In canter work, reward the horse by coming out of collected work into medium canter. The horse begins to look forward to cantering medium out of collected work so you build up a pattern of always having more horse in collected work, rather than having to struggle to maintain the collected work. Then during a test you can say (for example in a canter pirouette) "Now we will go medium canter" and then "No just kidding stay collected" and you still have a lot of horse there to access.
For engaged flying changes, supple the horse in the collected canter then let go of him just prior to the change. Use your seat by your strong position and even swinging into him (think shake out your shoulders) to supple the horse with the saddle.
An exercise to supple the canter: from shoulder fore position in canter, think of riding even a slight renvers fore on the quarterline, but with small amount of angle. Amazingly Rijkens can maintain this exercise with a huge amount of angle! But less angle is sufficient and easier for the horse.
In the extended trot, David asked Andrea to let the reins go, even flop, a bit. This had a freeing effect on Rijkens' shoulders while the rider continues the engagement with her strong position.
Rijkens saddling for his clinic lesson with David Blake.