Go to 'My DocumentsMy GamesBorderlandsWillowGameConfig and create a backup copy of the file 'WillowGame.ini'. Then using a text editor program change the following lines to activate the corresponding effect.

Cannot Drop Below 1 Health Point:
Set 'bDemiGodMode' to 'true'.

Increase Maximum Inventory:
Set 'InventorySlotMax_Misc' to a higher number.

NPCs can damage each other:
Set 'bIgnoreNPCFriendlyFire' to 'false'.

Bosses Do Not Regain Health After Dying:
Set 'bBossesRegenHealthOnReset' to 'true'.

Resurrect ALL Players When Someone Dies:
Set 'bResurrectAllPlayersWhenOneDies' to 'true'.

Shoot Own Teammates:
Set both entries of 'bIgnoreFriendlyFire' to 'false'.

Weapon Slot Capacity:
Set 'WeaponReadyMax' to 2 to 4.