Writing a great script for a short film is hard. Most people feel like you have to lengthen the script to create an in depth story, which isn't true. To make the most of your limited pages and minutes use these ideas:
1. Create complex characters.
Before or during writing, ask yourself some questions about your characters. Asking questions like "What was the worst moment in your characters life?" might seem silly, but it could lead you to some very interesting conflicts and plot points in your story. If anything questions like this will help you to make a world around your character it makes sense for them to exist in. Here are some character questionnaires to start with.
2. Don't neglect your set.
Though most of us are just happy to get a location to shoot in, remember that the location of your film can be a goldmine for planting character information and foreshadowing. Avoid exposition through dialogue by leaving a characters room a mess with clothes and make up before a date, or show the room as immaculate if your character is very controlling.
3. Make dialogue that sounds real.
People in the real world don't talk in full sentences, they sometimes stutter, pause, misspeak, use contractions. Read your dialogue out loud while writing. Do you believe someone would actually say it? Don't waste time with lengthy dialogue when you could convey the same idea visually.
4. Be confident.
Throughout this process realize that this is your vision and you are going to have to work hard to bring it out to the world. Be your own best advocate. Be confident that your idea is great and that it can impact something, and people will be willing to help. No one wants to work on a film that even the creator isn't excited about.