Start game as usual. During gameplay press [~] to open the console, then enter
 the following cheats:

 GOD 1        - God Mode
 UAMMO 1      - Unlimited Ammo
 GHOST 1      - Invisible
 FLY 1        - You Can fly
 NOCLIP 1     - Walk Thru Walls
 NOIMPACT 1   - ?

 Use the same code with a 0 to disable the effect.