I am not sure if I could have started this as effectively later in life, but when my most hoarding inclined child was small (3-5) I gave her the choice of helping/watching me clean out her room or letting me do it. Either way everything i removed would go into the shed for x amount of time and she could ask for it back. She quickly learned she couldn't bear to watch and let me go for it. And she's never asked for anything back. Now at 11 she both retains less precious paper to start off with and is learning to toss stuff when her room needs a big overhaul...
