Some people hate writing a synopsis so much they avoid agents who ask for it. I have bad news for those folks, chances are good that even agents who don't initially ask for one will want one if they take you on. So how can you take the pain out of this horrible process? One paragraph at a time.
It isn't easy to condense your hard work into a 3-5 or sometimes one page summary. It's even harder to do it and not make it sound like a stacatto scene by scene breakdown. Start similar to the way you started your query letter, with a hook sentence that sums up the heart of the book. From there hit on the important parts of the book. Infusing it with your writing style (or character voice) will help keep it from sounding like a scene by scene breakdown. Don't overdo the style though. The synopsis is about your story, not your writing. Finding that balance isn't easy, which is why most people hate doing one.
Know what the agent wants in the synopsis. Some want to know every detail, including the ending, some don't. Many agents preference differs on this. When in doubt, ask. Read the jacket of books. This is a good guideline to go by and will give you a feel for how the synopsis should flow.
Don't just take my word for it though, here are a few great links to Writer's Digest articles on the subject: http://writersdigest.com/article/10-synopsis-dos-and-donts