I think that it all starts with balance. Starting your cantering work can a bit scary and you will do some things defensively to stay in the saddle. Practicing your canter on a lunge line is not a bad thing. It's great for improving your seat. When you start improving your balance, then you will improve your relaxation in your saddle and follow the horse's motion better. Practice stretching your heels down at home. Pushing down on stirrups to get perfect heels can cause your seat bones to lose contact with the saddle and then you'll be all over the place.