I have to write a synopsis to submit to an agent. Are there any guidelines for writing a good synopsis?
Most important part is, reducing the story to the core plot. Some say you should only need three pages to summarize your story, some say one page should be enough. But if you can reduce its core to three sentences, then you are on the right track.
Impossible? Only three sentences? Imagine a friend asks you to summarize the Lord of the Rings for him. He never read one of the books or saw one of the movies. Do you really need three pages/one page to do it? Or can you do it with only three sentences? See, and LotR are three volumes, so you can condense your story, too.
Look at it from the agent's perspective. He gets 30 synopsis a day. He must flip through them and decide, which one is worth it. He cannot delay it, because tomorrow he'll get another 30 synopsis. Look at what you have written and ask yourself: Would I choose this, if I were an agent?
If the answer is yes, put your three sentences summary into the cover letter and your one page summary into the synopsis ;)
BTW: Randomman159's link says that the editors usually read your sample chapters first. That's not true. They read your cover letter first. Better make it good. And short.
A really useful website you might want to look at is
Sorry i can't really summarise :P
The accepted answer is reasonable, but I would add this angle. Don't write a synopsis of the plot, write a synopsis of the conflict. There can be a lot of business in a plot and it you try to summarize it, you may end up either missing the key points or skipping too lightly over too many plot points. The essence of the story is the desire that drives the protagonist, the things that frustrate that desire, and how the desire is achieved or how it is ultimately frustrated.
A wants B because C so he tries D and E and F and fails each time until he tries G and ultimately succeeds/fails while in the process realizing that H is really more important to him. Desire -> Frustration -> Triumph/Tragedy -> Realization. Summarize that way and you should capture the essence of your story.