First, run the game with the "-devuser" command line parameter. Then, during gameplay, press "~" during gameplay to bring up the console window. Then type any of the following cheats to activate the desired effect.
evileye - Bugs instantly killed when targeted
gun - Enable/Disable Gun Model Display
god - Invincibility
banzai - Invincibility, instant kill, and fast speed
speed # - Multiply game speed by #
ammo - Unlimited Ammo