During a quest you are about to complete, save by using a waypoint to "hard-save" your file (or use the "Save and Quit" option in your Start menu to hard-save your file, and then reload your file again), and then complete the quest WITHOUT saving in either of those manners.
Once you get your reward/EXP/gold, press Start, go to Options, and make a change to initiate a "soft-save." This type of save will only save your stats and inventory, but NOT your quest completion. Once that save is complete, simply quit the game without saving (such as closing the software from Home, or by tapping the Power button), and reload your saved file; if done correctly, you will be where your last hard-save left you with all the items and EXP/stats you had when you soft-saved.
You may repeat as needed for extra EXP/Gold/items; you can even use this glitch on *boss battles for the chance to obtain **any of their specific random Rare/Epic/Legendary gear.
*(This glitch only fully works on bosses that do not hard-save automatically after the battle is completed)
**(Glitch will not allow for item variance if the game is hard-saved after the boss chest appears, as once the chest appears, the items that drop from it are set, allowing for no variance in drops even if the game is reloaded; the glitch can still be used for repeated EXP, gold, and repeat items, however.)