Can't find any good guides or YouTube videos on this, so am going ahead and asking the SL community directly. Inventory mayhem!
You're the one who has to work with your own inventory, so it should make sense to you. Someone else's tips might not gel with how you view your inventory. That said, my method (for my 150,000+ inventory) runs thus:
I create sub-folders within the main inventory folders. For example, in the BODY PARTS folder I have (among others) the following: