Shoulder shimmies - start by just isolating your shoulders - lift each one individully, circles - backwards and forwards (if doing circles is hard start by rotating the whole arm, then bring into just the elbow then the shoulder - look at getting the back as well as the front; slide forward and back - without lifting (the reason why many people lift is they drive the movement from their upper traps - those big muscles up your neck - rather than the little ones in the middle of your back).
Got the movement - may take a while. Now build control - push one forward and back; now one forward, one back - in the shoulder - not twisting the torso.
Now smaller and faster.
If you ride a motorbike - riding is ideal for practice - butt is fixed and so are your hands (usually). Less useful due to the size of most steering wheels is driving a car.