My old coach had me practicing lots of walk to canter and halt to canter transitions in preparation for it. She said once you build up the hindquarter strength, you can get a much more balanced canter which makes it MUCH easier to try flying changes.
Halt to canter is alot of fun, and my boy can now do them easily off a voice cue.
How she usually taught them (I only know from watching) was to have you canter across the diagonal, trot for four strides over a slightly raised pole and pick up the other lead. Once you got that precise and down pat, she'd have you do 3 strides, then 2, and then simply ask you to apply the cue for the canter on the other lead right over the pole. The slight hop most horses do over the pole really seemed to help.