Personally, I would start be getting the horse listening to you by doing: inside bends/outside bends -this will loosen up the horse and also help with getting a nice outline if done properly!- once you have got a nice outline go forward in to trot-let the horse decide the pace for the first 10 or so strides (make sure you're still in an outline) and then do a half halt and get the horse really using it's back end-this will make the trot a lot more fluent and will feel and look nicer!! Then maybe try an extended trot but make sure the horse is really using it's hind quarters! To keep an even rhythm keep all aspects of contact equal, try not to pull back on the reins, don't lean/tip you're body and don't start flapping with your legs- the term less is more comes to mind! When exercising your horse transitions are essential to keep your horse happy doing work!
When doing a collected canter make sure you're horse is going forward and is not on the forehand..start doing a series of small squeezes down the outside rein, USE YOUR SEAT when you feel like your horse may decide to go down in to a downward transition do not start kicking as this will give un-clear signals and is not fair on your horse! Your horse should be in an outline to start with, but if it's not then it should go in to one once your doing collected if you're doing it correctly! When riding in collected canter you should feel like you're on a rocking horse!
I hope this has come in useful, if you have any other queries please do not hesitate to send me an e-mail, I'm more than happy to help!