On the title screen, press and hold 'G', 'FIRE' and 'LEFT & RIGHT MOUSE BUTTONS'.
Now use the following keys during play:
'F1' ------ Turn screen yellow
'F2' --------------------- ???
'F3' --- For 255 of everything
'F4' ------ Speed up the clown
'F5' ---- Speed up the enemies
'F6' - Flip screen upside down
'F7' -------- Flip screen back
'L' -------------- Skip levels
'M' ---------- Platform editor
While playing press the 'HELP' key for more jumps and shots. It won't show until you use one. It also shuts off the above cheat.