A technique I have been using for a few years that I borrowed from Kent Ford to exaggerate the torso rotation on the forward stroke is to practice paddling forward with your arms completely straight, with no bend in your elbows. This will no doubt feel odd, but the over-exaggeration will do you well. If you look down and can see your pfd rotating, you will know that you are rotating from the hips (which is good).
On the turning strokes (bow sweep, stern draw, etc.), I've found that looking at the stern of your boat when your torso is rotated at the beginning or end of the stroke helps to ensure you get that rotation.
Hope that helps!