This is what my trainer had me do; it may not be conventional (not sure) but it sure as heck worked miracles.
Ride DEEP into the corners. By deep I mean run your horse's nose into it until he's forced to turn or stop. Ride the outside rein, and release only when his nose is almost touching the wall. Apply some inside leg and have him bend himself around the corner. My trainer sets up cones in the corner and we HAVE to ride between the cone and the wall. I use no inside rein at all when doing this exercise; by now he bends automatically into the corners.
Do this at the trot as well. Ride deep into the corners. Then once going well at the trot, begin doing two circles in each corner, still riding as deep as you can into your corners. First a large circle, then a small one, in the corner. Be sure to plan ahead with your circles, and it sometimes helps to imagine your inside leg tracing the circumference of the circle and your horse bending around that.
When coming down to the trot from the canter, have him trot a very tight circle before you let him walk; this will discourage him from falling apart, hollowing his back, and getting strung out when he breaks to the trot, as in order to execute a tight circle he must collect himself up to some extent.
Hope this helps :')
Fly Without Wings