This tip is a variation of other tips, i.e. Writing is Circular, Writing is Messy, and others. The same concepts keep getting reinforced as I work.
Simply put, my rough draft has to be messy and rough because I keep moving things around. I often discover that something I’ve written at the very beginning needs to go towards the very end, and vice versa. An example: in the working draft of my unfinished middle grade, FLEABRAIN LOVES FRANNY, early on Franny says, “Of course you are just my imagination, Fleabrain.” The other day, more than 2/3 done with the draft, I realized that this simple sentence belonged in a much, much later chapter, and that this switch was important.
This is exactly why it’s vital for me to just get stuff down on the page in the beginning. Nothing is usually wasted; early sentences, paragraphs, and even chapters often herald what is to come towards the middle and the end, and often end up there.