At Exbiblio, until we can find a good Mac version control system that’s easy for normal users to understand, we have a convention of keeping all old versions of most of our files, naming them according to the date.
I wrote a little utility which, if you drop a file or folder onto it, creates a copy based on today’s date and opens that copy. I find it a very hand thing to have in my Dock. Just in case anybody else does, you can get it here.