I need to finish my WIP before I tackle the synopsis. So I went searching for advice.
Marg Gills says to start with the main character and his or her crisis. In her article at writing-world.com, she says to include snippets of dialogue or quote briefly from the novel itself. Don't neglect to reveal the character's emotions and motivations, those points that explain why a character does something, but keep it brief. If the setting is exotic, inject a taste of it into the synopsis with a brief paragraph. This includes any background information that is absolutely necessary for the reader to understand the story. Build excitement as you near the conclusion of the story summary by using shorter sentences and paragraphs. The synopsis is a sample of your writing; it is a taste of what reading the actual novel will be like, so give it your all.
She adds, don't forget that one- or two-sentence story line, or the theme of the story. It should go in your synopsis, or in your cover letter. Editors and agents like having this distillation; not only will it pique their interest, but it's something they can use when presenting the novel to the buying board. It's also something you can use, the next time someone politely asks you, "What's your novel about?"
This sounds like good advice.