Enter the cheats at the in-game cheats screen.

All Multiplayer Characters - Left, Down, Left, Right, Down
Big Head Mode (Story Mode) - Down, Up, Left, Right, Right
Infinite Chi - Down, Right, Left, Up, Down
Invulnerability - Down, Down, Right, Up, Left
Dragon Warrior Outfit (Multiplayer Mode) - Left, Down, Right, Left, Up