It was author William Faulkner who said it first, and a whole lot of teachers of creative writing, film making, journalism and other kinds of storytelling have been repeating it ever since: Kill your darlings.
This does not mean that you should take a chainsaw to your loved ones, it means that you, in your writing, should cut to the chase and have the courage to get rid of the elements that you love so much yourself, but that don't really add anything to the whole - or, even worse, actually weaken it. Typical "darlings" would be clever turns of phrase, insignificant trivia, funny anecdotes that don't really relate to the question at hand etcetera.
Samuel Johnson has a similar advice, that he allegedly picked up from a tutor: "Read over your compositions, and wherever you meet with a passage which you think is particularly fine, strike it out.”
There is clearly a belief in these kinds of creeds that most works can only gain from being edited more strictly, and personally, I find that a significant amount of lengthy articles, books and movies prove that the words of Faulkner and Johnson can not be repeated too often.
Variation: Kill your babies (Thanks, Albert Herring)
Some attribute this quote, or the slightly different version "Kill all your darlings", to Mark Twain, and I have also seen some claiming that it was F. Scott Fitzgerald who said it. However, I have been able to locate one single source only referring to Twain, and none reliable attributing it to Fitzgerald. But please /msg me should you know the original source.