I may add to this post from time to time, as more things occur to me. We’ll start off with just one quick tip:
Never throw away your old files. Archive everything. Even the stuff you’re sure you’re never going to need.
This afternoon, while revising a chapter in my current WIP, I realized that my detective was about to start interrogating a suspect on a subject for which he no longer had a foundation. In adding power to an earlier chapter, I had replaced a not very exciting scene with one that had more tension. Unfortunately, the scene I cut provided the springboard for the scene in the later chapter. A certain character is a bit of a financial charlatan, and that’s important, because his financial troubles may have motivated him to commit murder. So the gossip about his feud on money matters mattered.
Fortunately, I had a supposedly complete 2nd draft (now two years old) on my hard drive. Grabbing the missing scene, fiddling with it a bit, and finding a place to put it took only half an hour or so.
Scrivener, which is the software I use for writing fiction, makes archiving easy, but I’ve been renumbering chapters and moving things around. Finding that scene in one of the dozens of Scrivener snapshots would have taken longer. I went straight to the draft I had exported from Scrivener in 2018.