Short story writing is much different from writing a full length novel for a variety of different reasons. The most obvious is of course that it is abbreviated, giving you less time in which to bring your plot to a resolution. In order to write a short story that will grab an editor's interest, there are a few key points that you will need to remember.
First, it is vital to plan out your entire story from start to finish. Work on your plot arcs and your character arcs ahead of time. In many cases, you will have only a few thousand words to tell your story. Short story writing is in many ways more difficult than writing a novel. You have to be able to tell an entire story in a short period of time and make it believable. By taking the time to plot out both arcs ahead of time, your task will be much easier.
Second, you need to find the focus of your short story. Is it character or plot driven? Are you covering one main event in the story or does it have a broader theme? Instead of getting overly ambitious with your story, focus on the little things and telling the story as briefly as possible. Avoid going into too much detail, or having your story drift into areas that are not important to the plot's resolution.
Lastly, if you have a story that keeps getting larger and larger, until you are unable to contain it in short story form, consider breaking up the plot into episodes. Demand is high for continuing sagas and sometimes short stories go beyond their original bounds. A short story can be a day in the life, or it can be a hundred years condensed into a few pages. The key is making it believable and making it compelling.