Well, just think of the short story as a leaner or miniature version of the longer stories and novels you've written - sort of a condensed book: complete with interesting characters and events woven into a tight and purposeful story line. The big trick is to plan your short story:
Focus
You'll need to focus on a narrow aspect of the many possibilities available to you for plot, setting, and characters.
Purpose
If you begin a mystery short story, then stick to that genre. Don't go off on a tangent because there simply isn't enough space within a short story.
Know your audience
It is extremely important to know for whom you are writing and have a fixed idea about how you want them to react to not only what you are writing, but you must have a steady idea about how you want them to react to your style of writing.
Develop your story idea
The words with which you develop the characters, plot, and setting must be chosen with the economy of scale in mind of the short story. The fewer words it takes to do the job, the better. If you have to, then think Hemingway!
Chronological order
Try to carefully plan the chain of events which will make up the story's action. You can't afford to lose the reader.
Ready to write
When you are finally ready to write the finished piece, first decide on whether you'll be using the first person (I went with her, then I bought the ring), or the third person (Randy went with her, etc.). Then develop your characters much more so with dialog rather than by mere description. This lends to more interpretation on the part of your audience. Everyone enjoys discovering or figuring out the story on their own instead of simply being told that what is happening. There is more reader involvement this way.
Now, give it a shot. And good luck.
