Actually, I already had this.
It was a matter of survival. With ownership of all of the stuff that being a household of 11 entails, there's bound to be some chaos in the management of it. And if I'm trying to reduce the quantity of that stuff, I have to set it aside very clearly or it is just going to get dragged back into the parade of laundry, toys, books, etc. that circulate in this home. We have a closet in our foyer where I keep boxes on the shelves for such things. I even manage to keep them sorted by destination a lot of the time. Once a box is filled, I mark it (if it isn't already labeled) and load it up into the van. I did need a couple of new, empty boxes, so I set those up today.
And I will just say, once you put your newly full donation box in the car, make sure you drop it off at your earliest convenience, instead of lugging it around with you to every play date, Mass, and grocery run for weeks. Not that I ever do that.
Do you have a way to manage "outgoing clutter"?