I've read lots of posts on how to write a synopsis (I have this one bookmarked for its ease of use, and this one is good, too, especially for longer synopses), and my handwritten ideas notebook is full of the starts of synopses for this novel (five of them, if you wondered).
It took me a long time to get to a draft that I thought was complete. At about 800 words, I was satisfied that it was short enough to qualify as a "short synopsis," and happy enough about the plot points it covered. I read it over several times, patted myself on the back for finishing it, corrected some sentence flow stuff, and sent it off for critique.
My synopsis went to two different people. One has read the novel, the other hasn't. Here's my tip of the day: always have someone who hasn't read your book critique your synopsis.
The synopsis I was so pleased with a few hours before was completely ripped to shreds. It was fantastic (for the book; not for my ego). "Wait, how can this happen?" "I thought they were there?" "Is this even relevant?" There is a temptation to get defensive and say, "Well it makes sense in the book..." When that happens, it's important to remember the point of a synopsis: to tell the story to someone who hasn't read it. If the person critiquing your synopsis is confused, Amazing Agent X will be, too.
The goal for synopses isn't to write pretty sentences or make the reader infer anything. Its goal is to quickly tell someone (an agent or editor, probably) what happens in the book. Your job is to make it obvious what that is, so make sure that people who haven't read it are clear.