Invincibility:
Hold Down/Left + I + II and press Run. Alternatively, hold Down + Select before the stage text appears. Release the buttons when your ship stops flashing to be invincible while your weapons are at level one.

Level select:
Press Select at the title screen to enter the options menu. Highlight the "Exit" option, then press Down, II, II, Left, I, Right, Select. A sound and red background letters will confirm correct code entry.