I'm trying to add to a manuscript I started a few years ago. I've deleted the ending and plan to double the word count. The danger of writing on an old book is that our voice changes as we improve and editors can tell if we piece together a book over time. You know the beginning may have a lot of editing but plain, while about halfway your voice flows and have texture.
The funny thing about this one book is it's the one to help my voice to change for the better. It's my only first person story so far. I found writing in first person freeing me from worrying about deep point of view. As in first person you automatically add that to the story. Hey, it is your heroine/hero's thoughts, feelings, opinions and visuals you're putting to paper.
With it being a book I've written, I had to work hard on remembering what had already happened. So I reread the book and changed a lot of scenes. Making them fuller and more emotional - I hope. Along the way, I wrote a short synopsis of each chaper. Then I wrote a sentence or question for each "string" of plot. They aren't the main plot between the heroine and hero. Those are easy to keep up with. They're the subplots I can't forget as they will most likely affect the main plot. For example, "Phillip: He found out Larisa is a Precious One. What part did he play in her kidnapping? How did he know where the demons and Fallen had taken her?" As you can tell, it's a paranormal.
This way, as I finish the book, I won't forget to answer these strings.
That's just one of the things I've learned to do.