Pause the game and then press B. When you unpause it, you will be invincible.
Press Start to pause the game, and then press A, B, and C at the same time to go to the next level.
At the title screen, hold A, B, and C on both controllers while pressing Start on controller one. If you do it correctly, you will hear a chime. Now start the game. Pause the game and press A in order to have your health meter refilled when you unpause the game. If you press B while the game is paused, you will get partial invincibility. Press C while it's paused in order to view the number of the current stage, which you can change by pressing Left or Right. Change to a different stage number if you want to be warped to that stage when you unpause the game.