Continue
When GAME OVER appears, press A, B, Start, and Select.
Invincibility
At the Title Screen, press Up, Up, Up, Down, Left, Right, Up, Start. You're now invincible except for when you fall off of pits.
Level Select
At the Title Screen, press Up, Down, Left, Right, B, B, B, B, Start. Select stages using Select and begin by pressing A.