Im also in agreeance with the above posts.
Having your horse respond to your aids is the most important thing, him 'running on' while your asking for the canta can be a unbalanced thing, and you bouncing all over the place doesnt help!
To get him to be responsive, you just don't have to work on the trot to canta, work on walk to trot, trot to walk, trot to halt etc. Ask once, if he/she doesnt listen apply a artifical aid OR kick (as long as kicking doesnt throw you off balance).
Let us know how you go