I try to create a folder on Dropbox for each watch I purchase and just throw anything relevant into that folder. The name of the folder is generally descriptive enough so that I know which watch it refers too (although not so much for watches where I have had many duplicates). So there will be photos, original invoice and listing info, service records, extract info, etc. If I sell the watch, sales info goes in there and I change the name of the folder to indicate that the watch was sold.
I think that if I tried to be too organized and uniform, it would never get done and things would slip through the cracks. At least this way, all the information is findable about each watch, although sometimes I may have to dig through the folder to find it.
I am occasionally haunted by all the boxes, parts, and accessories stored separately, not to mention spare links removed from bracelets. I label them ... but still, it would be easy to sell a watch and forget. In fact, there was one time I sold a watch to a forum member, then a couple of years later he sold the watch to another forum member and that second buyer contacted me to ask me a question. When he contacted me, it jogged my memory that the watch had been serviced by the manufacturer and they had returned the original crown. So the crown was eventually reunited with the watch.