Continue game play:
Hold A and press Start at the game over screen.

Invincibility:
Hold Left + A + B on controller two and press Up(2), Down(2), Start on controller one at the title screen.

Adjust difficulty:
Hold Left + A + B on controller two and press A on controller one at the title screen.

Level select:
Hold Left + A + B on controller two and press B on controller one at the title screen.

Level skip:
Hold A + B on controller one and press Left on controller two.

Invisible bike:
Hold Left + A + B at the title screen.