Level select: Press Up, Down, 2, Left,
Right, 2, Up, Down, 2 when
the "Start" and "Configure" options appear
at the title screen (before the Alien 3 logo disappears).
Unlimited lives: Start game play in two player mode and obtain a high
score for both players. Intentionally have one of the players
lose all their lives and enter ALIEN as a name on the high
score screen. The corresponding player will have unlimited lives
when game play continues.
Extra ammunition: Start a game in two player mode and intentionally die,
then switch to single player mode.