I was never really a fan of outlining, but when I found the outlining feature on Word I decided to try it.
Outlining has saved me!!!!
I, of course, outlined Lily's Tree.  It wasn't until I got to the ending that I realized I had no idea what order the events of the ending of the book went in, which made the outline very good.  I can now consult the outline whenever I'm writing, and know what has happened, what is about to happen, and what order it happens in.
The outline is also nice, because I'm basically telling the whole story without backstory.  I don't need to explain anything in the outline because I understand the whole thing.
I saw Harry Potter and the Half-Blood Prince at midnight!  It was wonderful.  Here's a video: