The mare I have at the moment is currently being reschooled to go eventing. She is incredibly green on the flat and the one aspect I really struggled with was to get the correct canter lead especially on the right rein. I had lunged her and so knew it was not a physical issue as it was correct every time on the lunge, I was also pretty sure it was not me as worked really hard on my position and fiddled with a few things.
Jo May gave me a brilliant tip and it was to put a slanted pole in the corner. The high end needs to be on the side they are struggling to get the correct lead on (on the right if right lead issues etc). You approach in trot and ask for canter over the pole. 9/10 they give you the correct lead.
Yesterday we used it really successfully on a hunter who just got faster and faster when being asked for canter and could not give the correct lead (it was probably the first time he had ever been schooled). Over the fence he soon got the idea and about 5 minutes later was able to give the correct lead without the pole to help.
Its such a simple tool but teaches the horse in a positive and friendly way.