Defeat the Elite Four in the Johto Gym Leader Castle. You have to win the Castle with a team of your own, with no rentals or registered Pokemon. When you win you will be able to teach one Pokemon on your team a past move that they forgot or never learned. That is how you can get Pokemon such as Mewtwo to learn Confusion, or Zapdos to learn Peck. You can do this as many times as needed.