1) When you’re ready to stop, stop

For the nonfiction writer, the simplest way of putting this into a rule is: when you’re ready to stop, stop. If you have presented the facts and made the point that you want to make, look for the nearest exit.
–William Zinsser, On Writing Well (1976)

2) Your ending must do four (maybe more) things

Your ending must do four things: signal to the reader that the piece is over, reinforce your central point, resonate in your reader’s mind after he or she has turned the page, and arrive on time. The very best endings often do something else: They offer a twist that readers don’t see coming, but that nevertheless strikes them as exactly right.
–Bruce DeSilva, from Telling True Stories (2007)

3) Art needs to resolve, even if life doesn’t

Finding resolution is a gnarly problem. If the end of an essay is too emphatic it kind of undoes all the complexity you’ve set in motion. And yet the art somehow needs to resolve, even if the human questions are unresolvable, and I actually like that gulf and maybe even look for it – the tense standoff between the art’s satisfying completion and the impossibility of tidying up life itself.
–Charles D’Ambrosio, New Yorker interview, November 24, 2014

4) Authentic endings hint at another beginning

The most authentic endings are the ones which are already revolving towards another beginning. …To me there’s something false about an ending. I mean, because of the nature of a play, you have to end it. People have to go home.
–Sam Shepard, Paris Review interview (1997)

5) Close the parenthesis you didn’t know was open

To feel an end is to discover that there had been a beginning. A parenthesis closes that we hadn’t realized was open).
–James Richardson, “Vectors: Forty-five Aphorisms and Ten-second Essays” (2000)