1) Describe. Description is good, and you should always do it as much as you can. We know you see the world you want to show us, but we need to see it as well, every detail. Make good use of adjectives and adverbs. Metaphors, allegories, and references are your friends. When something happens, make sure all relevant questions of who, what, where, how are answered (unless, of course, it's a mystery). And during particularly influential events say, the introduction of a new character or setting, bringing to light a new concept or perspective in a dynamic character anything story-changing should be described as much as possible.
The van came to a halt at the corner of forty-third and eight, invisible aside from the strip of quarter-moonlight glinting off its mirrors. Four men stepped automatically from it, all dressed equally darkly, and equally silent. Not a word passed between them as they assembled in the alley and waited, straight-backed and still. Time ticked on by.
Of course, over-description is possible, but when you're writing a short story, you really don't need to worry about that, ever.
2) Be mysterious. So many people are so proud of the beautiful creation of their protagonist that they give everything away in the beginning. He's this tall, weighs this much, has this belief about women, has a deep fear of heights, went to Nova Scotia when he was six, likes this sort of clothing, and, of course, is a bloody god at this, this, and this. Not only does this bore your reader to death, but it removes any sort of development you could have done throughout the story, not to mention it makes your character look like a big fat Mary Sue, whether he/she is one or not. So, instead, If you must tell us about your awesome char, just give us the basics. Stuff one would notice right away maybe he's really tall, or he doesn't look like he belongs in wherever he is (clichéd examples, but you get my meaning). Don't give away everything, and certainly never say anything about a character's personality outright. Your readers should learn that through the story, not you telling them!
3) Use proper formatting. You don't know how horrible it is to read a story that's one giant paragraph. And while you may have outgrown that, there are other nuisances that even very top-heavy writers still throw into their work.
-Indent paragraphs. Please, for the love of God. If you're posting in some place that doesn't let you indent, double-space at a paragraph.
-Don't spam commas. Chances are, if you see a ton of commas in a sentence and you're pausing after every second word, you're doing it wrong. Know what a comma splice is, as well as when to use a comma and when to use a semicolon.
-Each speaker gets his own paragraph. I cannot believe how many writers do not know this, but when you get one paragraph with three different speakers, there is no way in Hell I am going to be able to discern who is talking without incurring serious brain damage. One person speaks. Enter. Tab. Second person speaks speaks again. Enter. Tab. First person speaks again. Enter. Tab.
-Finally, spell-check. Everything has one; Firefox has a built-in one.
4) Don't be boring. And I don't mean, "Make sure your plot is full of explosions and hot women." Contrary to popular belief, it's the style more than anything that makes prose exciting. Switch things up. Don't always have sentences the same length; add some compound ones occasionally. Avoid continually using the same word; I like to try to use particularly specialized words only once each in a paragraph, unless I'm using redundancy to establish hyperbole. Perhaps, use different types of words to start sentences (there's more to life than nouns and pronouns!). Use intriguing words: if someone tells you they don't like your story because they had to use a dictionary to read it, fuck them. They can go read Twilight if they want a wussy book.
5) Avoid giving away your story. There's a difference between foreshadowing and basically telling the audience what's going to happen. Although sometimes it's a good idea to tell your audience exactly how things will turn out, if the journey there is more thrilling and twisty than the result, you don't want to say anything that will automatically make your audience realize it's the butler who done it, so to speak. Foreshadowing should be very, very veiled. Symbolic to the extreme, only noticeable to the reader who finds out what happens, and then goes back to find that there was a hint all along. Foreshadowing embeds itself in the audience's mind and teases them, but doesn't give them the answers.
6) Know your action! Everyone loves a good action scene. So many writers think "EPIC BATTAL" when they think of action, but battles tend to wear thin very quickly. That's why no one likes Dragonball Z. Action can be a scene of passion, a scene of intense contemplation, a scene involving a stand-off between two characters without a clear outcome. Basically every strong turning point in a story can be considered an action scene, and while not all stories have these some like to slowly progress, letting things develop passively the ones that do need to have these turning points come out very strongly to the reader. During these scenes, use powerful language: strong adjectives and metaphors can really do the trick. Make sure to switch up your sentence structure rhythmically I like to describe it as a roller coaster. Through the scene, the little bits building up, you have short sentences often very short sentences that keep things fast-paced. These are the parts that don't really matter; they're just building suspense, like the little ups and downs of our roller coaster. Then, during big turning points in a fight, a mortal injury, or the first true embrace of love, the breakthrough realization of the antagonist's intent comes one or a few long sentences. These pop out at the reader and contain the strongest language yet. Some authors use this to torture their reader (particularly in scenes of torture), dragging things out with terrific descriptions that nearly make the audience cry out for mercy. Or at least a period. This is your big dip.
7) Don't think of your story as a series of events. A lot of writers just give the readers a blow-by-blow account of what happens, and it just comes out seeming unrealistic and hollow. Have a few branches, even just tiny ones. Although the definition of a short story is one with a single plot, even they have little nuances in them, bits that don't really relate to the problem at hand but serve as a bit of a distraction. And the reader doesn't know whether or not they're relevant. Good writers do this frequently to change things up a little to switch gears, so to speak. To keep the audience guessing and keep things fresh. If you don't do this, your story will be boring.
8) Clichés. I don't want to say, "Never use clichés, they are of the Devil," because that's not true. Sometimes clichés can be useful, usually when you want to give the audience a feeling while doing very little work. However, clichés are just that: they're so overused that they really don't mean anything anymore. George Orwell, writer of such famous books as Animal Farm and 1984, stated in his own list of do's and don't's, "If you've heard it before, don't use it in your writing." And I think, minus the exception I listed above and those made at the discretion of the individual, that this is a good, succinct rule.
9) Mood. Mood is incredibly important. In the first tip, I said to describe a lot. This is building on that: not only should you describe a lot, but you should do it properly. "Frolic" is not a bad word. It's certainly better than "play". However, it's a word you'd prefer to use only in certain conditions
say, a play swordfight between two children as opposed to a blood-driven battle between two generals. Similarly, what would be a better word to use when describing blood flowing out of a car wreck and sinking into a dirt trail: mingle or fuse? It's not always discernable, but each word has a sort of flavor in the mouth, and in the ear and mind as well by relation, some stronger than others. This is a good thing to consider when trying to evoke a certain mood in your audience.
10) Be satisfied. If you don't adore what you've written, neither will others. Thus, only publish when you are certain you've done your best possible work on this piece, and you can't work on it any more. If you don't, when you get criticism, you'll just get stuff that you already know you should have done but didn't do because you wanted to get something out there. Read and reread and rereread your work so you know it's as perfect as is possible for you. On a related note, don't talk about what you think is right or wrong in your foreword. Don't go "well this isnt my best but its ok i guess tell me what u think" with or without n00bsp33k, this is a huge turn-off for your audience.