For the following cheats you MUST be facing ABSOULTE NORTH! (Hang on a cliff??)
Level Skip
Go to the inventory and then to load game press and hold L + I + F + T.

All Weapons
Go to the inventory then to the large medipak press and hold C + T + R + L.

All Items
Go to the inventory then to the small medipak press and hold A + L + T + G + R.

God Mode
Go to the inventory then to the flares press and hold F + I + R + E.