You may be interested in trying the Hydrus program. It lets you tag images and also search by tags, kinda like a *booru, but run locally. It has been assisting me in posting to the Shana thread.
http://hydrus.x10.mx/When I save things to folders, I try to group them as I download. If from 4chan, I usually group it by series/character, or some general category like "catgirls" depending on the thread. If from a booru, it's usually by artist. I don't really suggest piling all your images in the same folder and hoping you'll sort them later. At least in my case, it never works out.
