My horse really struggles with canter transitions upwards. He tends to react to the leg a stride too late and its not as round as it should be as he uses that stride where he ignores the leg in order to hollow and come slightly against the hand.
I was given an excellent tip which was to tap him with the schooling whip in the outside hand when you want the canter transition and to do this a couple of times (it has to be sharp and get a reaction). At first the result was a bit messy but after a couple of times the transition became smoother and more reactive. I was able to then just ask with my leg and had an instant result which resulted in a smoother transition and no hollowing.
This is something I will be able to take into the warm up for dressage and will hopefully make a huge difference to my tests as this is where I drop marks at the moment.