I write a lot of flash.
I also sell a decent amount of it. I haven't counted how many, but I'd guess about 2/3 of my sales are flash-length fiction, which is defined in most places as 1,000 words or less. My flash stories have been published in markets including Daily Science Fiction, Flash Fiction Online, Abyss & Apex and Every Day Fiction. My most recent flash sale is to Nature Futures.
I don't sell every flash story I write. Some are real stinkers and rightfully get trunked.
There are writers out there who sell more flash, and those who edit or read slush for magazines that publish flash-length stories, and they're probably more qualified to talk about flash than I am. I'm going to do it anyway.
For starters, I think flash is underappreciated. Example: The Science Fiction and Fantasy Writers of America, also known as SFWA, won't let you in as an associate member with the sale of a flash-length story to a qualifying, professional market. Your sale must be a minimum of 1,000 words.
Considering the SFWA rules, you'd think writing flash was easy, that anyone can do it, and that's why it doesn't count. Flash seems to have that kind of reputation, but it's wrong.
Flash isn't easy to write. No, let me revise that: Good flash isn't easy to write. It's the same as for any length of fiction. Anyone can pound out 5,000 words of crap, but writing a good short story takes skill and practice. I might go so far as to say there's an art to writing a good flash piece.
The challenge of writing flash is, of course, telling a complete story in a few hundred words. You need a compelling premise. You need characters and setting. You need an inciting incident, rising action, a climax and a resolution. That's all the things you need in a longer piece, but you have to do it in a very compact space.
How do you pull that off? Here's my advice:
Keep it simple. Two characters. Three at the most. A plot with one major conflict to resolve. A setting that will be familiar enough to readers that a few well-chosen, well-placed words can serve as your setup. If you get into complicated world-building, chances are the story won't work at flash length.
I read a good analogy once, though I don't remember where, which means I can't give credit where it's due, but here it is. A novel is when the writer invites you into their home and you get to peer into all the nooks and crannies. A short story is when you take in the house by standing in the doorway. A flash is when you stand outside and peer through the window.
But simple doesn't mean boring. No matter what length you're writing at, you need to hook your readers. You need an interesting premise, a compelling story.
In a flash fiction, you can't beat around the bush. No complicated setup. You have to get to the hook fast. Your inciting incident should come no later than 100 words in. That doesn't give you permission to tell instead of show simply because it's more efficient. Prose is important. So are correct grammar, punctuation and syntax.
A lot of flash stories end with a twist. I've ended some that way. If you use a twist, make sure you foreshadow it. However you end the story, closure is important. You need a resolution.
My last bit of advice: No cat stories. I've heard this time and time again. The slush readers will thank you.