Good idea to share some tips about this!
I assign codes to all projects, invoices, quotations etc. and since about a year I also code all CAD file names and separate file formats.
Then I use Ccleaner to keep my computer clean and occassionally filter duplicate files.
I keep a separate folder for all external files, files to be published on web/print/digital media/television etc. these often contain duplicates.
Then I use cloud services and external HDDs as well.
Make sure to keep at least one extra HDD at another location, for example a safe deposit box.
-Ralph