Consider the shape of your story.  Is it a coin flipping? A puff of smoke? A spiral in or out?  A cell tower triangulating? Is it a boomerang, a fish-hook, a U-turn, a lighting bolt?  Pick a shape that mesmerizes you. Draft a story from there.

Compose a story on paper, in sharpie, with doodles, and no cross outs.  Take up a single page with images, words, designs. The page can be small as a post-it, large as a poster.

Voice memo a story.  Dictate aloud into your phone or computer or voice-recorder.  You may not jot down. You may not pause. You must tell it beginning to end.

Tell it backward — afterthought, aftermath, aftershock.  Write a story that begins at the end of the action and moves backward.  (exercise by Stace Budzko, from Field Guide to Writing Flash Fiction)

The first word of your story will also be the last word.  The last word of each sentence must be the first word of the following sentence.  Where you can, for the sake of surprise and variety, try to use the repeating word in a different sense, such as fall, the name of the season, and fall, the verb. (from Bruce Holland Rodger, from field Guide to Writing Flash Fiction)

Draft a story in exactly 1000 words.  Cut it to exactly 500. Then cut to exactly 250.  This can be the same story or 3 consecutive stories.  (Optional/challenge: Halve until you can halve no more.)

Use only one punctuation mark, preferably incorrectly.