Some horses just have a big stride and it can make it seem like they're cantering fast when really, that's just how they move. If this isn't the case for your guy, here is what I suggest:
Relax your horse. Are you in his mouth as soon as you ask him to canter (or before)? Answer this question honestly to yourself - I'm not asking if you're hauling on his mouth, although that is a question worth answering as well, I'm just asking if you anticipate the pacey canter and tighten your reins (maybe even subconsciously) before you even start to canter. Contrary to popular belief, the reins are not brakes, and don't influence your horse's speed as much as you'd think. Canoodling with your horse's mouth at the canter (keep in mind the nose bobs at the canter and your hands should likewise) creates a barrier and makes your horse tense, in which case you will NOT achieve a pleasant stride (at ANY gait).
Get back to the walk. Go in the arena and walk the fence on a loose rein. Yes, a LOOSE rein. Your horse's gait will adjust itself as he relaxes. Pick up a trot once you feel he is relaxed (head down, slower walk, licking and chewing, etc). Keep your reins loose, keep your hands steady, and make sure there isn't any contact for him to brace against. That's how a steamroller is born. Trot like this until you're in a steady, relaxed pace. The horse's head will start to drop, the back will come up, and you'll see licking/chewing on the bit. Pick up a canter with as subtle a cue as possible, and you'll see an impeccable difference in your horse's attitude and gait. Once you're able to establish this relaxation, you can start adjusting your contact on the reins to bring the nose in. This is all being said assuming the horse already knows how to yield to the bit and can flex laterally.
Last edited by Equilove; 10-25-2011 at 10:24 AM.