Be concise. This doesn't necessarily mean your writing has to be dry and lack description (I'd actually rather see purple prose in a short story than a novel, since it's over quick). This means you need to make every sentence, sometimes even every word count. Even if it doesn't move the story forward, make it interesting. Creative description, word play, varied sentence structure. You don't want any fluff in a short story. You don't want it in anything but a report with a page requirement really, but it's especially vital to make everything matter when you only have ten (or however many) pages to say it.
Write your story. Read through it. Read every sentence out loud, and ask yourself "Is this needed?" "Can I rephrase this?"