Writing a screenplay, like any process you control the pace of, benefits from a set schedule. Even if no one is standing over your shoulder enforcing a strict deadline, set a goal for when you would like the script completed and work a little toward that goal each day. Don't give in to the temptation to put off writing just because you can; this only results in procrastination.
If possible, build your movie script around topics you are well versed in. This gives your screenplay added credibility and boosts your confidence while writing. Of course, depending on the type of script you aim for, you may not have this luxury. For instance, if your screenplay is about the exploits of a government spy, getting firsthand experience is unlikely. In these cases, give yourself a solid foundation of knowledge by putting in heavy research before writing.
Typos, basic misspellings and other common writing blunders effectively kill any chances a movie script has of achieving any success. Editors in the film industry are inundated with screenplays from various writers every day, and the quickest way to ensure yours ends up in the wrong pile is to submit a script that doesn't show evidence of an exhaustive editing process. Any screenplay with fundamental errors appears unprofessional to the editor, and therefore has no chance at generating interest.
Never underestimate the importance of good dialogue in a movie script. Even if you think your screenplay does not rely heavily on dialogue, make a concerted effort to write what little there is well. Well-written dialogue will add considerable value to an otherwise average movie script. If you have any concerns about how the written dialogue translates from paper to the movie screen, hire professionals to read your script out loud in a practice run.
Not every joke you write in a screenplay is going to impress the audience. Humor is relative to the viewer, and what you find comical may have no effect on others. If your script is riddled with numerous failed attempts at humor, chances are your screenplay quality will suffer significantly. Test your jokes by sampling them to others before you submit your script. If you aren't getting laughs from the people who proofread your script, chances are editors won't laugh either.