The trick is to have another pair of exactly the same pair of shoe laces inside your pants. Tie one of them to the center of the other and hold it with your hand in your pocket. Your shoe is tied all the time but it is not visible to the audience because of your long trousers that cover the knot. As you shake you leg, pull the shoe lace you hold in your pocket and then you pants to reveal the hidden knot.
Another thing you can do(recommended) is to take a rubber band instead of the other shoe lace and tie one end of it to the center of the fake shoe lace in your pants. Attach the other end with a safety pin to you pocket. Hold the rubber band stretched so the shoe lace is visible. As you start to shake your foot release the rubber band. It will pull up the fake shoe lace in your pants and you are ready to pull the pants and reveal the hidden knot!!
Practice a few times in front of your mirror to master the trick. Good luck!!