Pause the game and press Up, Right, Left. Then line up the icons to activate the different cheats.

Invincibility
Taser, Grenade, Key, Goggles

Unlimited ammo
Pistol , Rifle, Taser, Grenade

One hit kills
Grenade, Key, Goggles, Pistol

Invisibility
Goggles, Grenade, Pistol, Rifle

Level 2
Briefcase, Camera, Watch, Triangles In Circle

Level 3
Camera, Triangles In Circle, Two Circles, Watch

Level 4
Briefcase, Aim, Aim, Rectangle Chip

Level 5
Aim, Briefcase, Camera, Briefcase

Level 6
Card, Gun, Watch, Aim

Level 7
Card, Biohazard symbol, Two Circles, Camera

Level 8
Rectangle Chip, Rectangle Chip, Aim, Two Circles