There's nothing worse than writing a synopsis...is there? I haven't found it. My first piece of advice on writing a synopsis is...get someone else to do it :-). Seriously, as a writer, we love too much of our book to ever boil it down to its key elements. My other bit of advice is to write it before you write the book...that way, you don't know what you don't know yet :-). Of course, that's not always possible...either way, however, I have found a quick recipe for writing a short synopsis. I hope it helps some of you. It sure helped me.
The way things are going in the marketplace, most selling synopses are 5 pages or less, no matter what the length of the book. Here's how I do it. That being said, there are as many ways to write as there are writers, so take what you want and leave the rest.