I can't give you anything terribly specific as I suspect you ride Western?
But one that can be used cross discipline, is to ride leg yield on a circle in trot and canter. I start by trotting in a forward, relaxed trot (depending on horse's training, this may be more towards collection, or very open in a long frame) on a 20m circle. Leg yield the horse in off your outside leg, onto a 15m circle, then leg yield back out off your inside leg. As you get back to the 20m circle, ask for canter. Get your canter going forward and relaxed, and then repeat the leg yielding exercise in canter - leg yield off the outside leg onto a 15m circle, then off the inside leg back to a 20m cirle. Trot when you get back to the large circle, and repeat on both reins until your horse is getting perfect transitions in all cases, and is softly moving off both of your legs.
I've found this excerise immensely helpful in creating a more balanced transition as well as giving the canter more jump and engagement, plus its so easy :)