You can use the shape of the arena to make what you are asking clearer to the horse when schooling.
For instance, if I have a young horse having real problems with the canter strike off on a particular leg, after the usual back/saddle/teeth and myself checks, I would ask for canter going in to first corner of short side, e.g… on the left rein I would ask at M or K, and on the right rein at H or F.
Most horses will realise that they have to get round two corners on whichever leg they are on, and will make more of an effort to go on the inside lead. It just makes it clearer to the horse.
If the canter to walk transitions aren’t coming together well enough, asking for them off a 10m circle at G (heading towards C) or at D (heading towards A) has the effect of backing the horse off the wall.
Another good tip is to do something difficult at the same spot of the arena a few times, and then reward the horse if it starts to anticipate and sits more just before that spot.
All simple ways to make it clearer and easier for the horse.