I sort by content. Originally I had a double set of most main categories (CAS, BUY, BUILD), one permanent and one temporary, but these days I find that most of my cc stay permanently anyway.
When I dl new files, I'll save them to folders like "test in game", "test in CAS", "test in Build".
When tested, I'll move them to the main folders, and for most of those there are subfolders (CAS>hairs, CAS>clothes, CAS>costumes/uniforms, CAS>Jewelries etc.)
If I have many files in a main folder from the same cc creator, there might be a folder for that creator instead of using sub categories.
One reason for saving everything temporarily to the "testing" folders, is of course to test that they work, but also to edit tags, filename and cost etc. To make it easier to find stuff in the BB catalog, I try to use the price tag for sorting similar content, ie a janitor's tools, office stuff, deco food etc. I also remove any CAS "situation" tag, or random possibility. I wish we could edit those tags and cost also for EA made content...