One nice thing about being a modern day artist is cameras. Specifically camera phones. They allow us to take progress pictures as we go through the process.
This is extremely handy with my current painting. I can compare and contrast prior editions of the painting. Such as the aspects of the eyes and nose that previously attired the essence of the person. But after working on it tonight those features were inadvertently painted away.
But because I photographed them both I can see what those specific features were and add them back in.
I find this very helpful. Not to mention it detours the frustration that develops when you took a painting just one stroke too far and changed it.