Press Start during game play. Then press Up, Down, Up. Down. The message "Enter cheat code" will appear at the top left. Then, enter one of the following codes to activate the corresponding cheat function:

Extra armor
Enable cheat mode, then press Right, Left, Right, Down, Up, Right at the cheat code prompt.

Invisibility
Enable cheat mode, then press Up, Right, Left, Down, Down, Up at the cheat code prompt.