From a discussion I had earlier this week, some thoughts about how to keep a flash story from growing into a full-blown short story. (Of course there are plenty of good flash stories that ignore these tips; they’re meant for people who are used to writing longer stories and are having trouble coming up with something very short.)
One problem I see a lot is that the flash is just a summary of the plot of a longer story. Sometimes it’s summarized to the extent that there’s no dialog at all. Every time you write a sentence of summary is another hint that maybe your story isn’t flash.
There’s also world-building summary. Secondary world fantasy and SF settings that need a lot of worldbuilding are hard to keep short. I keep trying anyway, and sometimes I succeed. But if you have to plunk a couple hundred words of exposition into your story for it to make sense, it’s probably not going to work for me.
Limiting the number of characters helps too. In 1000 words there isn’t a lot of room to develop multiple well-rounded characters.
If you’re really struggling to keep a flash story short, try picking a story that can be told in one scene. It has to really be one scene though, not one scene plus a ton of backstory.