A reader has different expectations from a flash fiction piece and a short story, even though both fall under the broader category of short fiction. Short stories may focus on character development, but flash fiction must be driven to the end by plot. A real, tangible ending must be achieved. In my experience reading and critiquing short fiction, I hold higher standards for flash fiction than I do for short stories: the flash fiction writer must employ economic word use by telling the story in the least amount of space possible to achieve his or her desired effect on the reader. However, I will be more lenient when it comes to character development. Sometimes the use of tropes is unavoidable--the reader being situated in the story and in the motivations of the characters as soon as possible can account for an easier reading experience.
Because flash fiction is characterized in its brevity, you can't waste any words setting up the story! Start in the middle of the action--don't worry, your reader will follow if you do it right. Don't insert irrelevant details about the setting as your plot unfolds.
Make sure, though, to not introduce too many characters. Description and complex characterization, in many flash fiction pieces, isn't even there. If the story is between two characters, there might not even be names involved. Don't forget to make good use of your title to situate your reader.
Flash fiction is usually less than 1000 words, but I have seen wonderful pieces at less than 250 words. The better you get, the less space you'll need to develop a stellar story. If getting under 1000 words is hard at first, overwrite. Use as much space as you can to fully develop a story arc, and then go back to trim your wordage.
End with a bang.