I'm with Wild, I've seen some pretty silly advice on this subject. Forget the runway thing, nobody in real life walks like that. It isn't so much about pelvic width either, although there are significant physical differences to take into account. Keeping your shoulders still is almost guaranteed to make your torso stiff and make a natural swish nearly impossible. When you practice putting a soft side-to-side sway in your hips, you'll find that a relaxed torso and letting your shoulders rotate front-to-back slightly makes the swish much easier and natural. Straight (but relaxed!) posture and keeping your elbows close to your side also facilitates a swish way more than you might think. Practice practice practice, and when you find the sweet spot practice even more to build muscle memory. Before long it will start coming more naturally.
