Write a brief outline that breaks your plot into three distinct sections: beginning, middle and end. Identify the central conflict and climax of your plot; this gives your characters impetus to act.
Identify the main characters in your story and develop them. For a short story, no more than three characters are advisable; any more and you will dilute your opportunity to flesh them out and present your reader with strong and compelling personalities. If you are struggling with character development, take a brief break from the main story to ask yourself some questions about your characters as though they were flesh-and-blood people. What is important to them? What are their relationships to the other characters in the story, and to their surroundings? What journey do they make from the beginning of the story to its end?
Make strong stylistic choices to define your writing style. Decide whether to tell your story in first person: "I shuffled across the street, shading my eyes against the harsh sunlight," or third person: "Hank walked across the street, shading his eyes against the harsh sunlight." Do you prefer a spare, pared-down style like Ernest Hemingway's, in which adjectives and adverbs take a backseat to emotional intensity and pacing? Or do you want to describe every detail as D.H. Lawrence did, so richly that the reader can almost reach out and touch your characters? Your style will help you economize your words and effectively communicate your story.
Pick up a thesaurus. You may find that in the course of writing your short story, words such as "said" and "went" appear frequently. If so, develop your plot and strengthen your writing skills by excising these simple words in favor of more precise and complex ones. How did your character feel when she spoke? Happy, sad, angry? Instead of just saying something, did she gush, wail, or shout it? If your story describes a series of events, how did the characters get from place to place? Did they walk, skip, slide, drive or dance? Be creative!