I'm a graphic designer, so for me everything is Client/YYMM-Project/_FINAL/YYMM-COLLATERAL-NAME
Within each project there is a _PROCESS folder with a _ELEMENTS subfolder for pieces the client has given me to work with.
For invoices I do YYMMDD-ClientName-Project-Sum.pdf. When the invoice is paid, I rename the file to add -PAID- before the client name. Its simple, but its allowed me to easily track and maintain projects and billing over the years.
If I end up working for another 83 years, I guess I'll pad the year with a 0...
Proper file management is an undervalued skill and should be taught both in school and in corporate environments. In an old tech job we had a public folder on the server that was total chaos. So many people insisted on naming their files MAY-%day%-%contents%-%personsname% -- and, as you'd expect, people spent countless hours per year trying to hunt down that one file so-and-so worked on before they left for another job.