Three-letter codes
I save everything in My Documents to make it easier to back up that one folder and its sub-folders to an external hard drive.
Within My Documents and the sub-folders, I try to start every file name with a two- or three-letter code, like MGT for business management ideas, FIN for financial, TAX for tax, QB for QuickBooks, ML for mailing labels, etc. I have some duplication and I'm planning when things slow down to maybe consolidate all my documents into one folder and sort by the codes and see what duplicates I have and what I want to delete, and clean it up a little. For now, I'm pretty happy with the codes, and if I want to I can do a search with the codes and always find whatever I need. Under My Documents I have other folders: Maps to client's addresses, Music, Photos, QuickBooks Data, Tax Data, and folders for each client as needed. I try not to have individual files under My Documents--they all need to be in a folder. I feel like at least my computer is organized, although some other things in my life aren't. I use the codes for reference files, not actually for data files.
For data files, I start filenames with the date, in the format yymmdd so everything will sort automatically by date, with everything for a particular year together, instead of using the mmddyy format, which doesn't really sort things by date, but by month. Took me a couple of months to get used to the yymmdd format, and I don't like it for other purposes, but I really did need some way to label and sort dated information, and finally just gritted my teeth and stuck it out and finally am comfortable with it.
Also I've re-named my frequently-accessed folders so the names start with a ! or @ so they will appear at the top of the directory listing in windows explorer and in file open windows, so I don't have to scroll through the directory listing so much. At one point I had ten or so folders beginning with those characters, and the top of the foler listing was getting crowded, so I suggest if you try this you just have five or six.