A great exercise for working on establishing a good seat in downward transitions is as follows:
Start in the trot, on a 20m (large) circle. Spiral the circle in so that you have between 3-5m between your track and the wall, make sure you aren't making the circle too small though. From here, work on leg yielding the horse out slowly using your inside leg to move over and outside rein to keep the horse straight and regulate how fast you're going sideways. Part way through your yield, ride a transition to walk and keep yielding.
Try the whole exercise just in walk first to make sure you and your horse understand the yield. After you have mastered the exercise, then every time you want to walk, think about the yield wherever you are in the arena, but don't actually do it.