Okay, I have no idea why my unbalanced green horse and I have no problems cantering (on the lunge mind you.. maybe that's why!) but I find following the motion really easy and keeping my horse from speeding off too.
But what helps me is not to lean forward.. you sit up and your eyes are up! The canter is a very elliptical movement for the horse. If you imagine a marble starting at the cantle, rolling down to the pommel, getting airborne, going at an arc towards the tail and then landing back on the cantle.. that is the motion of the canter. Your hips need to follow that motion too.
But the difference is, you'll need to drive them forward, and when your horse has that moment of air, your hips will follow it as long as you keep your weight down in your legs and seat!
If you drive yourself forward with your hips, your legs and knees play a HUGE role in keeping balance. You need to stretch those knees down and put weight into that seat and those legs.
Remember all energy, at all gaits, needs to escape down your body and flow out of the soles of your feet. If you pinch with your knees, your body will curl and you'll lose a stirrup, feel unbalanced, and your horse will be further unbalanced. Also you need to put your weight to the outside. Not all of it, but more of it so you can better support the horse's shoulders from caving in and help them stay upright.
Also.. a huge thing that I forget to do is half halt before the canter to balance, and before you pick up a trot, to re-balance. Yesterday my horse and I tripped really badly (he landed on his nose.. basically a front end dive) because I forgot to half halt. The strength of the half halt in both seat and leg varies on what level the horse responds to. At first, I really had to half halt with some strength and eventually it got softer. But it only got softer because he responded quicker.
Again.. eyes up, chest open, roll those shoulders back (thanks everyone for pointing that out to me) and follow with your arms.
Good, luck.. have fun! :)