Short stories are like miniature books. However, unlike novels, you don't have the added luxury of taking the time to let the reader get to know your character. You have to get the invested quickly. Therefore, your short story content should only be in regards to advancing the storyline.
As a writer, it's your job to get the reader interested in what you have to say. However, because most people make up their mind as to whether or not they want to read your work within the first few sentences, you have to get them hooked quickly and then reel them in just as fast. In longer works, you can spool out the character's backstory, what makes them tick, and employ all kinds of nifty plot points to enrich the reader's experience. When writing a short story, you have to do the same thing, only in a fraction of the word count.
Consider making your characters as dynamic as possible. Make a surly, frustrated character enraged at the world. Make a sad or depressed individual possibly suicidal. In short, make your antagonists the most hateful person on earth, while making your protagonist positively glow. That's not to say that they can't be flawed heroes. But be sure that their flaws are gigantic so that the reader spots them immediately and you can move on.
As for plots, they need to move and they need to move quickly. Your character can't stop in the restaurant on the way to resolving some crisis and spend a few pages thinking about his dilemma over a cup of coffee. The action needs to keep coming, but it needs to come in a way that drives the story ever closer to its conclusion. Every sentence you write you should ask yourself, "How does this advance and enhance my short story content?" If the answer isn't obvious, chances are it isn't a sentence that you need to include.
Finally, the conclusion needs to be a satisfying one. This is the same for any written work. Obstacles need to be surmounted, plot points need to be resolved and loose ends need to be tied up. You can even throw a nice, neat bow on the top in the form of a life lesson or by illustrating how your character has grown over the course of the story. The important thing is that it needs to be wrapped up, and wrapped up tight.
This is what you need to write a successful and satisfying short story. It's not simple. The hardest part, much of the time, is deciding what needs to be in there and what you can do without. But, with practice, it becomes easier to make those kind of decisions. When you become proficient at it, you can begin employing those skills in your other works. And when you write a novel utilizing this formula for a short story, then you have a book that keeps the pace spry and the readers hooks. And isn't that what every write wants?