At the end of a level, while the next is loading, hold down 'LEFT MOUSE', 'DEL' and 'HELP'. The screen should now flash, and you can skip a level by pressing a key.
Pause the game and type "IREM". This will give you invincibility and will enable the following keys (possibly need to hold 'SHIFT' while typing):
'N' -------- Advance to next level
'B' - Advance to the next main guy