My storage isn't full yet, it hasn't ever been completely full.
Slot A has inventory items, StreetPass items, fireworks, fruits & plants, and other "handheld" items. It's almost full.
Slot B has all of my clothes; Hats, Accessories, Shirts, Pants, Socks, and Shoes. It's about halfway full.
Slot C has all of my furniture and junk. It has flooring & wallpaper, cafe items, DLC, and house plants I don't have room for. It's a little under halfway full.
I think they key is to keep organized!