This might sound totally ******ed but it worked for me ...
I rode from more of a "racing" point of view so when I got back into an English saddle, when it came time to canter I was bridging my reins, leaning over the pommel and out of the seat. It took me a couple of weeks and some reading up to a) stop myself instinctively leaning forward, b) keep my butt in the saddle and c) really ride my horse forward with purely my seat (as opposed to using my legs and/or hands on his neck).
So, what I did was "sat" on my tail bone. This automatically made me sit up straight and prevent myself from leaning forward. I have really good leg placement from racework, so that was never a problem that they might go forward.
The second most important thing is to understand the horses movement, expecially when travelling on a circle or into corners. Our natural reaction, or what MANY of us have been taught at ridding school, is to turn your shoulders to where you want to go. WRONG. In the canter, forget your shoulders and think purely about your hips. It's hard to explain without pictures ... When a horse is cantering on the right lead, his left hip sits behind his right hip and you need to match this. So your right hip(inside hip) should sit slightly in front of your left. Because your are moving more easily with your horse, it becomes easier to sit to his canter.
I really hopes this makes sense, haha. I hope it helps. :)
"Neigh, neigh, nicker, snort." - Pony