You might think this is a NaNoWriMo-only rule, and you would be wrong. Wrong, wrong, wrong.
When you write, your brain is working. Sometimes it associates ideas that don’t belong together. Maybe you realize at some point that, cool as laser beam-blasting unicorns are, they don’t belong in the current MS. Your first instinct may be to delete said unicorns.
STOP RIGHT THERE. DO NOT HIT DELETE.
No, what you want to do is Copy or Cut. (Cut is a command that looks like Delete at first, but actually saves what you Cut and Pastes it into a new place. You can literally think of this like a piece of paper being cut and moved versus being copied or trashed.) Both of these commands will give you a copy of the unicorns.
Now, Paste the unicorns into a separate documents. Some people give each deletion its own document, some have one big doc for all the things they delete. Some people keep a master file for the ideas they really, really like.
(Or, you can be like me, and save a new copy of the document every few days so you can always look back and find it. I have dozens of copies. Don’t be like me.)
Point is, you never know when you’re going to think to yourself, “This needs some laser blasting-unicorns.” It could be the same project, or something entirely different. Maybe you like the unicorns so much that you initiate a new project around them. And won’t you be glad then that you saved everything!
One day you may want to look back on the unicorns out of a sense of nostalgia, or to see how you’ve improved. (Those unicorns shoot PLASMA now. Duh.) Future you will be really glad you kept those around. (Future you might also want to kill it fire, but at least you have the reassurance of first thinking it over for a long while.)
So, never delete your writing. Squirrel it away for a rainy day. There may be gems among the moldy acorns.