I just did a search on "how to write a book synopsis" and got 24 million hits. You will find them varied, variously interesting, and ultimately about as helpful as "how to write a book".
One thing they all have in common: it's hard to do.
Susan Dennard's advice, complete with Star Wars example, is a great way of thinking about it. Even if your novel is more complicated than a movie or YA novel (which are shorter, with cleaner plotlines, than an epic fantasy can be) I found it a great start to thinking about streamlining and cutting to the core of the story.