Step 1: Designate a place in your home to store future sale stuff
The point here is that when you finish a book you are not going to lend out - throw it in the box. Can't find the saucer for the cup? - put the cup in the box. Aren't going to wear that sweater again? - throw it in the box. Just bought a new iron? - put the old one in the box. Did you receive a gift you are never going to use? - in the box it goes. Do your really need 3 watering cans? - put at least one in the box.
This serves two purposes - it keeps clutter down and keeps you organized.