Uniform boxes are the whole game
Same-size boxes stack like bricks and waste nothing; a unit of mixed-box chaos wastes 30% of its volume on gaps. Buy the medium and large moving boxes in quantity, fill them to a consistent weight, and the unit packs twice as densely — the single highest-leverage storage decision.
The load plan
Heavy and seldom-needed first (deepest), light and possibly-needed last (nearest the door), with the aisle decision made before loading: no-aisle units pack tighter; aisle units cost ~15% of area but keep the unit usable. Sofas on end, mattresses vertical at the walls, shelves assembled if possible — vertical is free volume.
The inventory photograph
Photograph the loaded unit (front to back, box by box), tape a copy of the list inside the door, and keep a digital copy. Six months later, the photo answers "is the tent in storage or the garage" in ten seconds instead of a 40-mile round trip. The best-organized units are all documented; the worst are all memory.