Mindy asked in the comments section about working with a green horse who falls out at the canter. A horse who falls out on a shoulder at the canter will drift towards the lazy side, creating a weak shoulder and a weak gait. It is not pleasurable to ride a horse in this habit.
This is an interesting problem because it is often the rider's posture which creates a horse that 'falls out' on one shoulder or another. Generally for a horse to fall out on the right shoulder, they will be on the right lead and you will be cueing with the left leg. Often the rider sits too deep with the left hip and has inadvertently moved the horse over. For some horses this becomes habit. For other horses though, one side is weaker than the other and horses are inherently lazy and will cater to the weak side.
No matter the situation, a good fix is to focus on keeping your seat bones right next to each other, not 'pumping' with your seat, and keeping one hip from drifting forward. At the same time, you can provide a counter balance with the rein on the offending side. So, you may ask for the right lead (this works best traveling in a straight line in an open space) Then focus on your hips, you want a level even seat. Steady the horse with the right rein- squeeze with your right hand while bringing it down and in and low by the withers. This will create a minimal bend in the neck and will bring the shoulder up under the horse.
Checkout Glenshee Equestrian Center for a good series on rein aids. The Indirect rein of opposition in front of the withers is similar to what I describe. The illustration is an exaggeration of what I described also. If you imagine the horse in this illustration is cantering on the right lead and drifting right, you will be able to straighten him by forcing his right shoulder back underneath him.
Also, Mugwump has a series going right now about cantering, leads, and the one sided/crooked horse. The comments on the entry Balance are useful.
Good luck and I would love to keep up to date with you or anyone else who is having a similar problem.