Annie Dillard:

One of the few things I know about writing is this: spend it all, shoot it, play it, lose it, all, right away, every time.  Do not hoard what seems good for a later place in the book, or for another book; give it, give it all, give it now.  The impulse to save something good for a better place later is the signal to spend it now.  Something more will arise for later, something better.

The Writing Life, p. 78-79