Go to the options menu and enter your name as "GO POSTAL". Exit to the Game Select screen and quickly press Left, Right, Up, Down, X, Z, Y. A cheat menu will appear where you can choose FAST RELOAD, DAMAGE BONUS, and WEAPONS from the options menu.

FAST RELOAD lets you fire missles rapidly. DAMAGE BONUS lets you kill aliens, generators, and vehicles in one shot. WEAPONS gives you unlimited firepower.