Hold the 'HELP' key and type "XIFARGROTKEV" or "XIFARGROTCEV" or "XIFARG ROTKEV" depending on which version you have. This will give you infinite shields and you may press the following:
'L' -------- to show a picture of Luke
'C' -------- to show a picture of CP30
'D' - to show a picture of Darth Vadar
number ---- to play the sampled speech
'RETURN' --------- to cancel the cheat