Here is my guide to farming for XP, Upgrades, Eridium and Cash. I use this mainly for the Upgrades and the Eridium which I've maxed out three times already which has allowed me to upload all The Black Market Items.
Notes:
You do need to have beaten the game.
If you are playing on The True Vault Hunter Mode, it will work for you too
Go to Hero's Pass
Go to The Vault of The Warrior
Enter The Vault of The Warrior
Go toward The Warriors Spawning Location
You'll reach the Checkpoint and Vendor area.
Continue on towards The Warrior
Once He spawns, Kill yourself by either letting The Warrior damage you or by jumping in the Lava.
You spawn at the Checkpoint shown above
Restock ammo if need be
Work your way back towards the steps - DO NOT JUMP OFF THE LAST STEP!
Stand on the last step - Fight The Warrior from this spot and you'll take zero damage.
Shoot The Warrior in his sensitive spots
Use a weapon that shoots far, a machine gun
Kill The Warrior and loot!
Once you've looted everything you can get - Press ESC
DO NOT GO TOWARDS THE WAYPOINT FLASHING ON YOUR SCREEN
Save and Quit
Press Continue
Now you're back at the Checkpoint - Sell your unwanted items and buy ammo.
Rinse and repeat until you max out on Eridium.
I've got two orange weapons so far and maxed out Eridium three times doing this. It's well worth it just for the amount of Erdium and cash that drops.