As a hunt rider going into dressage, I've had to change my seat very significantly. I naturally have a light (LIGHT) seat, which can be good, but really it isn't. Lol
My instructor tells me to "clean the saddle front to back with my butt". So instead of "sinking into your hips" (never heard that before?" canter WITH the horse. You're probably stiffening up which also will cause you to bounce around.
So when you're riding, loosen up your back, but be able to tighten your abs. Push your rear to the very back of the saddle, then sweep it forward with the motion of the gait. Does that make sense? I know when I first started, I realized I don't have hardly any core strength. It is hard work!!
What discipline do you ride? I'm assuming its hunters/jumpers? You won't use such a deep dressage seat like I just explained, but it should get you on the right track to get a nice, sweeping seat!
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman