You have to post with your hips, not legs. So relax your body and just try to move your hips only. The way you can practice is getting big exercise ball (from Walmart), sit on it with your legs spread (not BETWEEN the legs really but closer to the "edge") and move your hips like you are riding a horse (the ball will bounce imitating the trot). Your legs should just stay in place, but you'll get the idea on how to move hips.
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