Unlimited lives:
Press Up, Down, Left, Right, Right, Left, Down, Up on the Joystick at the title screen before the high score screen appears. The screen will flash to confirm correct code entry.
Unlimited lives (alternate):
Load or reset the game, then execute the following BASIC command before running or restarting the program.
POKE 8868,173
View ending sequence:
Press S + P + E + C + I + A + L + F + X.