Something that could help you is while keeping julia and katelyns points in mind. Having someone else lunge your horse while you ride. It may sound silly as something you usually just do with beginners, but it allows you to focus solely on your position, and what you are doing. You will be able to hold the saddle and feel how your seat is naturally tilted while at the canter. You can then actively make those movements, not being afraid to exaggerate. From explicitly making those movements, they will eventually come naturally over time, just like posting or following with your seat at the walk.