Some of the things that makes a successful short film...
The film is put to good use as a visual medium.
The Writer/ Director also should demonstrate a personal connection to the story. This helps establish a strong point of view, that isn't as common as it should be.
They focus on one character, or couple's journey, but no more. Because a short film is not a feature, it doesn't have much room for subplots, secondary characters, or montages. It should cover a single dilemma, and it should resolve it in some way by the end.
They use available resources. It makes everything easier, and doesn't get the audiences hopes too high, so that you don't run the risk of disappointing.
They include memorable characters.
There's a twist at the beginning and a twist at the end. That way the audience is kept on their toes.
These are just a few of the main guidelines of what to do if you want to make a successful short. Written by someone who has experience as a short film critic.