Collect and Sort Everyone's Upstairs Junk in "Crap Baskets"
Nobody wants to walk up a flight of stairs to put away every little thing. Craft blog Sew Many Ways suggests giving every member of your family a "crap basket" to put their stuff in.
The idea is so simple it seems obvious once you think about it. Each person is responsible for their own basket and they just carry it upstairs whenever they go. It also helps teach everyone to get in the habit of keeping things clutter-free by taking something with them when they leave.