In keeping it simple, I find that a focus on the direction of the kick allows for a stable hip position while enabling rotation in the trunk. Feel your kick moving up and down, rather than sideways. A sideways kick is generally a sign of too much rotation. And when the hips rotate too much, you'll lose support from the water and the hips will start to sink.
Practice skate, zenskate, and zenswitch with enough rotation to bring the shoulder out of the water while kicking up and down.
John, in my Masters practice I try to refer to the first portion of the session as "stroke tune-up" rather than drills.