Hold down 'ALT', 'CTRL', 'S', and 'U' when the activision logo appears and press fire. You should have infinite lives (except on level 3). This also stops the cable from breaking on level one.
Turn your AMIGA on, wait for the hand to appear, insert disk TWO, the hand will disappear, and then come back, now insert disk one. During gameplay press 'ENTER' on the numeric keypad to advance a level.