Focus on the story. The first and most important key to any good synopsis is to stay simple and true to the skeletal structure of the story. The synopsis is not the time to delve into the intricacies of subplots or talk about funny characters that make cameos in the screenplay. It is time for you to simply and succinctly explain your story in as clear a way as you possibly can. Focus on the protagonist, antagonist and any ancillary characters that only deal with the moving forward of the plot. If they like the story, which is the most important thing, then they will read and fall in love with your script.
Stick to your act structure. As most scripts follow a traditional three-act structure, so should your synopsis. Start with a short paragraph that gives any background info the reader will need. This can be a short description of the given circumstances at the beginning of the film that might include protagonist description, local description or (in the case of a period piece) description of time period. The next paragraph is for act one, where you set up the inciting incident that throws the protagonist into action and ends with the act one climax. Paragraph three will be the longest and will describe the trials and tribulations the protagonist goes through that takes her to the act two climax. The final paragraph will describe the action of act three, where the final climax of the film happens and we find out what happens to the protagonist.
Keep it short. A screenplay synopsis should be no longer than two typed pages, in 12-point font, double-spaced. This is not very much room. It can be done, however, no matter how complex your movie. Working within these confines may seem challenging at first, but you will find that keeping yourself to this strict rule will help you to strip away the extraneous material that you are holding onto for no reason other than sentimentality.
Make it enjoyable to read aloud. The screenplay synopsis serves two purposes. One is for an exec to glance at and read easily in less than five minutes. The second is for you. When you go into a pitch meeting with an executive, he will want you to pitch your movie in five minutes or less. The screenplay synopsis you write can act as a kind of script for you to use to pitch your movie orally. Don't feel like you must stay true to the script, verbatim, but write it so it flows well when spoken. It will read better on the page, and it will kill two birds with one stone as far as serving you for your pitch meetings are concerned.