To enable cheats, you need to add -cheats to the game's command prompt. An easy way to do this is to right click on the game shortcut and choose 'Properties'. In the 'Target' field, add -cheats to the end of the line. Now press 'Apply' and then start the game.

When playing, press Tab to open console and enter the following cheats:

max life         Heals your characters
infinite ammo    Unlimited ammunition
finite ammo      Turns above cheat off
const life       God mode
no const life    Turns off god mode
kill enemy       Kills all enemies on current location
ai off           Turns enemy AI off
ai on            Turns enemy AI back on

Thanks to Revolution reader Daniel Grunder!