Here’s the big short story secret: set a deadline. Then meet the deadline. As several of my students have noticed, although you can write short stories more quickly than you can novels, you can also spend more time tinkering with them.
I’m ghostwriting three (yes, THREE) series of short stories and novellas at the moment. Although I write my first drafts quickly – remember, cr*ppy first drafts – I also tend to tinker. Then I need to mentally slap myself around the ear.