This is a brief entry today, but one that I've found to be really useful in my writing so far. When working on a long piece, or maybe even a moderately sized story, try using notecards to write down your story beats and chapter happenings.
I use plain, non-lined notecards and generally use one per chapter. This allows me to mix them up if I feel that a section of the book should come earlier or later, or, more often than not, remove one or more of the notecards when they do not fit at all. This way I can save the cards that I am not going to keep in my current narrative and store them in case they become useful in the future. There isn't enough space in the brain to store all the ideas we have at once, so why try? Paper is easier to go back to when I'm in a hurry.
Of course, when I'm in a pinch, I have used other pieces of paper besides notecards. I even once used torn apart popcorn bags for writing down story beats when I had a down moment on my dinner break at work! Give this process a try if you are a slow plot developer. It might just provide the fluidity you need to see the story through to the end.