Practice is key; nothing trains you for writing under pressure like an artificial deadline. The most important thing to remember about story is that you might have at least three major "parts" -- not necessarily in order, or defined into parts, but they must be present for a complete story.
Rising Action: set up a world and characters. This can be very short, with just enough information to get the feel across.
Climax: a mid-point where something changes. This can be anything, ultra-dramatic or subtle.
Resolution: wrap things up. This is absolutely necessary for the "feel" of a complete story; so many modern stories have no ending. Nothing is more frustrating than reading a story that just stops.
See here for more information:
And I suggest reading through the archives at:
Short science-fiction stories under 600 words. You can really get a feel for what works and what doesn't by compressing it down that far.