Beat the game. At each saving point save, and restart the system. 
Change the resolution after each time you do this. (note: you must 
start with resolution set on "high res. letterbox" then cycle up 
from there.)This will add a cheat menu at the title screen. There is: 

Invincibility - on/off
All moves - on/off
All items - on/off
No key renewing - on/off
Play as any monster - select
Play as Alien - on/off
Play as Pres. - on/off
Level select - 0-0