So for KS, I kept the bread and butter pieces (walls, corners, floors) in one big bin, and the decorative pieces (doors, columns, tables and chairs, treasure) in a small bin.
Cities, I grouped by type. Bin for streets, for roofs, for Tudor walls, for stone walls, for posts, for doors, etc.
Castles was a little tricky in places, but mostly the same - big towers, little towers, battlements, round battlements, walls.
Now - it feels harder to draw lines. The boundary between "basic building" and "decorative" is less clear. Do LED pieces go with the regular build stuff? There's so much of it, and I want it as part of my "phase I" layout assembly! On the other hand - it IS decorative, and I kinda want to keep it isolated so I can readily find a torch wall when I want one. Ditto magnetic. What about ramps and elevation pieces? Is that decorative? All the mini floor pieces, 2x1 or just the half-inch squares? Are those "decorative"? They're more like Advanced Builder Set pieces - more functional than ornamental, but at the same time more specialized than generic.
Still trying to figure out how to put this all away in a manner that's efficient and useful.