What makes a good synopsis? Well, these are just my opinions, but worth thinking about.
Be concise as you can for the length of the work. Three or four pages should be the limit, although it’s useful to include a general one paragraph summary at the start. Unless you’re doing a chapter by chapter breakdown, which can go on, but it’s useful to have the concise summary. This is all general—if you can, make sure you find out from the agent or publisher you’re sending it to. Everyone has a preference.
In this first paragraph, compare your book to others which are selling well—this will help the editors (more importantly the marketing department) understand how to sell it. That’s the most important decision at the back of their mind. Make it clear what it is you’ve written. Think of this as a brief sales pitch. Don’t compare it to something too obscure, or something that has bombed recently. This comes back to market awareness. Just write about what happens in the book, what the characters go through. Keep it simple. After you write everything that happens, take out all useless commentary. Be brutal. Make sure all that you describe is key to the story, nothing more.
Like writing, it’s not an exact science, but it’s useful to bear this in mind. And it’s no guarantee this is absolutely correct!