It's no secret that I have problems with brevity. The sweet spot lengthwise for short stories, for me, is usually between six to eight thousand words, over the word count limit for many markets. I do well with flash fiction, but that's a different way of thinking. If anything, I expand one liners into a story. Jokes where the punchline isn't necessarily funny.
If I want to keep a short story in a more limited word count, I have a specific strategy. I conceptualize around a single scene: one point of view, a specific unit of time and either the same setting or a continuous progression - for instance, someone walking around a city. If I narrow my focus to that range, I find it much easier to kept the story succinct.
Not to say that it always works. Occasionally, I've formed the broad outlines of a tale, only to find that it spins deeper and wider, even within that snapshot of a moment. My brain thinks in big tangents and tangles, and I can't always rein them in ... at least not and end up with a complete story.