Confuse your Legs
While seated, lift your right foot a few inches from the floor and then begin to move it in a clockwise direction. While doing this, use your right index finger to draw the number 6 in the air. Your foot will automatically turn in an anticlockwise direction and there's nothing you can do about it!
The left side of your brain controls the right side of your body and is responsible for rhythm and timing. The left side of your brain cannot deal with two opposite movements at the same time, so it combines them into a single motion.
Try this with your right foot and left hand and you should have no problem!