During gameplay, press ~ and enter any of the following "Codes", then press Enter:
Code -- Effect
GiveMeAll -- All Cheats
GiveMeFuel -- Full Fuel
MoreLives -- Full Lives
HealthMe -- Full Health
GiveMeInvulnerable -- 30 Sec. Invulnerable
GiveMeInvisibility -- 30 Sec. Invisibility
GiveMeAllAmmo -- All Weapons