My "secret", which is easier said than done, for writing a good query letter or synopsis, is pretty simple. If you've written a novel that has a good story, well-told, telling the agent about it doesn't help. You're playing the lottery when you do this, and while you may get a hit, you'll probably get a bunch of rejections for each hit. I cringe when writers say you can expect ten, or twenty, or fifty rejections for each request for a full. You shouldn't expect any rejections. No agent or editor ever rejects something that smells like money.
If the query or synopsis is, in and of itself, a good story, well-told, the agent or editor will ask to see the full manuscript pretty much every time.
A good query letter or synopsis is a story, it must display your writing talent, your storytelling ability, and your character building ability, if you want a very high percentage of requests for fulls.
I follow the hardcover book jacket synopsis model, primarily because back when I first started, two editors told me that was exactly what they looked for. It's never done me wrong. You still have to do it well, of course, but if agents and editors are looking for a good, well-told story, you need to give them one.