Unlimited lives:
Type [Apostrophe] C [Apostrophe] at the high score screen. 

Unlimited lives (alternate):
Load or reset the game, then execute the following BASIC commands before running or restarting the program. 

POKE 11312,128
SYS 2560 

Unlimited lives (alternate):
Load or reset the game, then execute the following BASIC commands before running or restarting the program. 

POKE 11387,173 

Invincibility:
Load or reset the game, then execute the following BASIC commands before running or restarting the program. 

POKE 5346,173