I highly recommend Shoot Ogawa's Master of the Muscle Pass DVDs. Not only does he teach the move in exquisite detail, slowly building the difficulty level, but he also teaches some excellent applications for the move that aren't just the standard falling-up effect. As Steer and Rick have said, the muscle pass can quickly become just a novelty for other magicians to appreciate if used how most people use it, but I think Shoot's teaching really opens up a lot of possibilities.