Study past science fiction screenplays. These screenplays will give you an idea of the screenwriting process and average length of a science fiction feature. The screenplays will also showcase story lines and plots that have already been used. You do not want to get a rejection because your script is considered a "Matrix" or "Star Wars" rip-off.
Plan out your screenplay fully. What science fiction elements need to be explained in the story? Are you using different galaxies, and if so, what needs to be described? Picture the movie in your head. You may understand the plot, but will the viewers comprehend the story you are showcasing? Many science fiction plots like "Gattaca" and "The Matrix" may be hard to understand at first, but movies like "Alien" and "Predator" are straight forward sci-fi films.
Focus on character development. Many science fiction films offer an exciting plot, but ultimately fail because they do not focus on enough character development. It does not matter how much action a movie features, if character growth does not occur, the audience will not care. Incorporate the science-fiction elements into the plot. For example, in "I, Robot", the character was very suspicious because of the robots' behavior. How will your world and its science-fiction elements affect the character?
Turn the genre on its heels. Write the unexpected. Science fiction has so many cliches that one of them can be turned around to create a compelling enough story that will garner viewers.
Choose a sub genre to go along with the science fiction. This doesn't mean your story has to be locked in the two genres, but it will help move your story forward. For example, you can write a science fiction movie with horror elements like "Red Planet", comedy elements like "Back to the Future", or romantic elements like "The Astronaut".
Check the logic of your story. Just because it is science fiction, it does not mean you can fabricate things that are not scientifically correct. If you want to reference something that does not exist, simply create a short scientific explanation or scene to make viewers believe that the technology can in fact be invented.
Write with vivid details, but do not add too much description because the reader will get bored easily. Use the most succinct adjectives to keep the story moving along. For example, instead of describing a futuristic city and all the technology, focus on a few pieces of technology that impact the plot, like flying cars or teleportation. The other details are for the set designer to fill in. For example, the detailed worlds from the "Fifth Element" are not written completely in the script, merely vague details. The production team later will give the audience a clear view of what the whole futuristic city will look like.