Pokes
Infinite Lives
POKE 29375,0
This removes the command;
29375 DEC (HL)
Which is used to decrease the player life counter held at 26352.
Aliens Don't Fire
POKE 31591,201
Just RETurn immediately from the alien subroutine.
31591 RET
Immunity
POKE 27832,0
27831 LD A,1
27833 LD (26276),A