When the pink loading screen appears, hold down Caps Lock and type "IMAGINE".
When the opening bit plays, type "PETEJOHNSONWANTSCHEAT". Now, when you press S, it causes both exits to open, and you can go on to the next screen without finishing it.
Or, type "DALEY88" instead at the pink screen, and you can now restart at the level you last died on!
While still on the title screen type in "TUESDAY 14TH" and when you press the fire to start the game you will be presented with an options screen that will allow you to obtain infinite lives, etc.