Level select:
Hold A + B + C and press Start at the title screen.

Unlimited continues:
Return to the title screen after losing the game. Press Left(3), B(3), C(3), Start.

Reversed controls:
Hold A + B on controller two and press Start on controller one.

Additional missiles:
Press one of the following controller actions, depending on the current level, before meeting the refueling plane:

Level 3, 13, or 16
Press Left + B.

Level 5, 11, or 21
Press Right + B.

Level 9 or 19
Press B.