Few things are more frustrating than being unable to get back to something you’ve been satisfied with in your work. That’s why you should save your project files as copies, while incrementing version number whenever you make some significant changes to it. Many software has this functionality built-in to save you couple of clicks.
Another question is how to distinguish different versions, because, clearly, you don’t want to make another copy of your file literally every time you want to hit ctrl+s. Well, it depends on your personal workflow. When you look at your project at current state and you feel that this state – is something that you might want to roll back to, or fork the project to be able to explore different look (or setup); then you create another version.
Beside version number I personally also use snapshot numbers (as you can see on the picture above). Shapshot – is something that you can’t call a full-fledged version, but some tweaks and edits you made is enough for you to back up your previous work. As I said – this is all mostly on personal perception level.
And, of course, version numbers are always usefull to make personal notes, something like “Version 2-08 has better lighting”. And, needless to say, it’s super-usefull when you collaborate, and you and your colleagues can quickly refer to a specific version of some shared asset.