Infinite Lives
POKE 25903,0
Removes the instruction which would otherwise decrease LivesDisplayCharacter.
25900 LD HL,24499
25903 DEC (HL)
255 Lives
POKE 24499,47
POKE 25931,47
This changes the code which sets the starting lives from ASCII 51 ("3 lives") to be ASCII 47 (which is "0 lives" minus 1).
So it cycles through an entire byte before reaching 0 again.