When I got my warmblood he could not canter worth a crap. Not even when I would play with him at Liberty in a round pen. I never saw him canter out in the pasture. When he would canter, he would always cross fire. Turns out he had some big chiropractic issues going on, so after a couple adjustments he was much better.
I basically started on-line first, asking him to canter so I could identify his weakness, be it unconfidence or unbalance. For him, it was a combination of both. I worked out the confidence issues first and then introduced him to Karen Rohlf's ground exercise called "finding the sweet spot." I HIGHLY recommend it!! I started at the walk, then progressed to the trot, then eventually the canter. I also worked him on hills to build balance and strength and when riding I was very patient and used a figure-8 pattern to set him up for his leads. I also asked him to find this "sweet spot" while riding as well.
His canter is now LOVELY! It's so nice and smooth and balanced. That's what has worked for me.