Keep it short. Although short films can technically last anywhere from 30 seconds to a half hour, there are disadvantages to longer products. Greater duration can sometimes equally higher production costs, for example. Aim for a story that can be wrapped up in no more than 10 minutes. Hold especially firm to this standard if you're considering entering your film in festivals.
Craft a first page, and last page, that rivets attention. Ideally, your film should open with an image or event that emotionally engages the audience. Viewers should feel placed in an authentic environment with characters they can understand and empathize with. Many short films end with a whimper, not a bang. Avoid this pitfall by closing your short film's script with a touching final moment.
Eschew elaborate plot structure. There's simply not enough time for serious character and narrative development in the short film form. Your film's protagonist should have an extremely strong need that is made clear as soon as possible. Just as soon, an obstacle to fulfilling that need should be mounted in the script.
Aim for comedy. Weighty dramas are best reserved for the longer form of the feature film, where actors and a crew have the time and space to develop pathos. Funny stories, however, can make for winning short films.
Write your story with narrative efficiency in mind. Eschew exposition and lengthy spans of dialogue. Focus on verbs, and refrain from adjectives and adverbs when possible. Actionable images should be the focus of your story, not description.
Use screenwriting software, such as Final Draft, to properly format your short film's story. The proper form is necessary for a director and crew to understand your film in terms of it actually being shot and produced.