1. Have a clear theme.
What is the story about? That doesn't mean what is the plot line, the sequence of events or the character's actions, it means what is the underlying message or statement behind the words. Get this right and your story will have more resonance in the minds of your readers.
2. An effective short story covers a very short time span.
It may be one single event that proves pivotal in the life of the character, and that event will illustrate the theme.
3. Don't have too many characters.
Each new character will bring a new dimension to the story, and for an effective short story too many diverse dimensions (or directions) will dilute the theme. Have only enough characters to effectively illustrate the theme.
4. Make every word count.
There is no room for unnecessary expansion in a short story. If each word is not working towards putting across the theme, delete it.
5. Focus. The best stories are the ones that follow a narrow subject line.
What is the point of your story? Its point is its theme. It's tempting to digress, but in a 'short' you have to follow the straight and narrow otherwise you end up with either a novel beginning or a hodgepodge of ideas that add up to nothing