The answer is simple: trot, trot and more trot.
And then more trot.
Add some hills, also at the trot.
Better jumping and better canter comes from trot work not from canter work. Trot ground poles, raise every other or every third one to make the horse elevate and use their hind end more. And do lots of changes of directions and tons and tons of trot.
I trotted my horse for the whole first year I had him and when we finally did canter, it was awesome. Then this year getting ready for show season we did buttloads of trot work, and finally in the show ring for jumpers, his canter was AWESOME!
The answer is trot! :)