On the level/player selection screen, move the mouse to the crystal
at the top of the Apocalypse logo. Imagine the mouse buttons are
numbered like this:

'SELECT' 1
'MENU'   2
'ADJUST' 4

Holding down various combinations of buttons activates a different
numbered cheat mode, according to the sum of the held down buttons.
The cheat mode levels run from 1 to 7, although 5 to 7 are the same.
As the cheat levelgets higher so the better the cheats become. On all
cheat levels you can select any planet to start at. At cheat levels 3
and higher the shield charge increases, at levels 3 and higher you
have rapid fire, and the gun temperature coole ricky to select any
particular level, so a way to get cheat level 5 (the best) is to
press [Z] to continue instead of [SPACE] when you insert the levels
disc.