Press P to pause game play, then activate the following cheats by entering the corresponding codes. Note: This may require v1.4.1 or later:

CHEAT FUNCTION - CHEAT CODE
Multiple balls - multishoot
Mega shoot - megashoot
Smaller paddle - shrinkpaddle
Bigger paddle - expandpaddle
One ball - settoonestrike
Fire ball - fireball
Wall - thewall
No monsters - removemonsters
No bad bonuses - removebadbonues
More balls - addballs