Include the ending
For fiction, follow the main chain of decisions and consequences through to the resolution. Reveal key turns and explain what changes for the central character.
Show the structure of non-fiction
Explain the order in which readers encounter the ideas, and what each major section contributes. A short annotated chapter outline can make a complex argument easier to follow.
Revise for a reader coming to it cold
Ask someone unfamiliar with the work to read the synopsis once. If they cannot explain who drives the story or what the argument builds towards, revisit the sequence before adding more detail.