Currently editing (3) different manuscripts for writing buddies, and I Looove this process. Yes, it's time consuming, yes, it forces all my mental faculties to go to work, but there is no better way to improve on my own writing. Guess it's that whole "teacher becomes the student" phenomena that comes into play. Whatever it is, I'm grateful for it!
Also, I diagramed out both my finished manuscript and its sequel-in-progress to the "Hero's Journey" made famous by Joseph Campbell (and reiterated in "The Writer's Journey" by Christopher Vogler). This helped clarify where the stories are weak and offered specific ways to enhance them. A really cool excercise and one I highly recommend. (Vogler's "return with the elixir" is quite literal in my first book....which I'm going to consider a good thing!)