And having had a painful experience of going back to something after a major interruption (job change, house move) I now try to be a bit more formal in the way I handle the manuscript.
Background
I tend to work in sub-files - thread, scene, chapter basis. In the first generation file there will be research (if needed), discussion of the scenes (if not simple), possibly notes on what could go into the scene, then finally the scene.
The next stage is a clean copy of each scene in a new file, or a number of scenes that flow together.
When these are at second draft, they are added onto the main manuscript.
Complicated but it works for me.
But working out where the heck you've got to after a longish down period is awful.
So these days I do a basic form of document control, just like I have to do for formal technical reports. At the top of each file I will write in date opened, purpose of the document.
Then each time I edit it, I put in the next date and a one-liner on the edits.
When I finish with a file, and say have copied scenes to the draft chapter file, I make a note in the document control section that this document is now for archive and put it in the archive folder.
I also keep an informal work diary - again a few sentences at most, which lays out a quick summary of what I've just done in a writing day, and what needs doing next. That way, if hit by a crisis, and have say a week or two down, then I can come back, read the work diary that says just worked in file x, next thing is .... in file y and go to file y. (Or possibly re-read last bit of file x.) So I don't lose the first half hour/hour back at writing getting my head back around the plot complexities.
Hope this helps a bit for the next time.....
