"Kill your darlings."
This venerable nugget of writing advice is often attributed to William Faulkner, though in fact the British writer Arthur Quiller-Couch said it first. In a 1913-1914 series of published lectures at Cambridge University titled On the Art of Writing, Quiller-Couch said:
If you here require a practical rule of me, I will present you with this: Whenever you feel an impulse to perpetrate a piece of exceptionally fine writing, obey it-whole-heartedly-and delete it before sending your manuscript to press. Murder your darlings.