Imagine the trailer. Begin with a punch, end with a flurry. Life and death situations are the most dramatic. There should be surprises and reversals within the major plot, and also within individual scenes. Once the parameters of the film's reality are established, they must not be violated.
Limitations call for interesting solutions.
Avoid mean-spirited storylines. Proper spelling and punctuation. No scenes off the spine, they will die on screen. Screenplay descriptions should direct the reader's mind's eye, not the director's camera. Begin the screenplay as far into the story as possible.