Honestly, the biggest mistake is putting cushions away while they're still damp. In an urban setup—balcony, rooftop, small courtyard—moisture and dirt are the enemies. First, brush off crumbs and pollen, then spot clean with mild soap and water. Rinse well and let them dry completely in the sun or a well-ventilated room. Don't rush this; even a little dampness can turn into mildew in a closed storage box.
Once they're bone-dry, decide where they'll live. If you have indoor space, a closet shelf, under-bed storage, or a storage ottoman is ideal. Keep them off the floor and away from damp walls. I like to slide them into a breathable cushion bag or an old cotton duvet cover—not a sealed plastic bag, which traps humidity. Add a couple of silica gel packets or cedar blocks if your apartment runs humid.
If indoor space is tight, use a weatherproof deck box or a storage bench on the balcony. Choose one with a lid that overlaps and ideally some ventilation. Line the bottom with a moisture barrier or pallet to lift cushions off the ground. Put the cushions in breathable bags first, then in the box. For very small balconies, a vertical storage cabinet or under-seat bench works well; just measure your cushions before buying anything.
For long-term storage, clean and dry them, then store flat or on edge rather than folded, so the foam doesn't crease. Check them once a month, especially after rain or humid spells. If you see any musty smell, air them out and let them dry again. And if your cushions are regularly soaked, consider quick-dry foam or solution-dyed acrylic covers—they're much more forgiving in city weather.
That's it: dry, clean, breathable, elevated, and checked. Do that and your cushions will come out looking fresh instead of like a science experiment.