What Kayty said just brought something to mind. I had a dressage clinic a week or so ago; my horse has the basics but he is still very green. The trainer, a local dressage judge, had me trot him up into a working trot, then half halt him down until he was just ready to walk and then trot him up again. It not only helped to rebalance him but it kept him off his forehand. It also got him to understand that when I add that leg pressure he is to respond NOW and not two seconds later. She said that my horse needs to be kept thinking so to do a lot of changes in direction, serpentines or half serpentines, lengthening and collection etc. The more we broke things up the better he did.
She also said the same thing as Kayty indicated..a lot of transitions up and down and changes within the gaits themselves.