Unlimited lives:
Keep holding [F5] during game play.
Unlimited lives (alternate):
Load or reset the game, then execute the following BASIC commands before running or restarting the program.
POKE 36156,173
POKE 32772,169
POKE 32773,127
POKE 32774,141
POKE 32775,13
POKE 32776,220
SYS 32768