Writing and directing short films force you to focus on what's most important about story. It teaches you to strip away anything that doesn't further the development of character and/or plot, and leave only the most essential elements. Learn these lessons by working on short films, and your feature film scripts will be that much more focused and engaging. 

When I'm writing short films, I force myself to write as little dialogue as possible. It bothers me when I hear dialogue that's too expository, or "on-the-nose." In real life, it's rare for someone just to blurt out exactly how they feel. It just doesn't sound natural. Why tell an audience what your character is thinking/feeling when you can show it? I really like the challenge of using visual cues to give the audience information about a character, a situation, a place, or a story element. For example, look at the opening scene of Training Day. We learn a lot about the character and his backstory simply by what we see on screen. This quickly gives the audience some context. 

Use the visual medium to its fullest potential. Think of ways you can tell your story simply through what happens on screen, and not by what people say to each other. Often, what a character does tells us more about his/her nature than what is said. I don't mean to say that you shouldn't use dialogue, but a film is made up of different elements (visuals, music, dialogue, etc.) and you shouldn't rely too heavily on any one aspect, sacrificing something else in the process. It's about balancing each to have a well-rounded film.

As you work on your short film projects, challenge yourself to tell the story visually. It's a great exercise.