Image Snatched from Deviant Art
In essence, the synopsis is nothing more than a very short summary of your entire novel. But you want it to be nearly as engaging as your novel, not a boring recitation of facts.
Starting with the basics -- although the length will vary from agent to agent, your synopsis should typically be two to three pages long. (To get to this length, hit only the major plot points and avoid flowery language.)
If your synopsis is one page, you can single space it with breaks between paragraphs. If longer than that, it should be double-spaced.
Stick with Times New Roman, 12 point font and 1 margins.
You last name/book title appears in the upper left hand corner (Harrell/Destined); the page number goes in the upper right hand corner (this info should be contained in the page header).
Capitalize each name the first time (but only the first time) that you use it.
Now more for the craft -- You should hit the beginning, conflict (action, reaction, decision), plot twists and the ending. Do not leave your reader hanging at the end with a what will happen? sort of question.
The beginning should include your main character's name and age as well as a hook (much like in your query). If where and when details are important, be sure to include them early on as well.
You want the reader to related to your main character right away, just as in your novel. You should use the same "voice" as in your novel (e.g., humorous, snarky, literary), but the synopsis should always be in third person present tense, even if your novel is in first and/or past tense.
You will not, however, want to (or have room to), introduce every character in the novel. Include only those who directly and significantly impact the main character. In the synopsis, sub-plots tend to fall away in favor of the overarching plot line.
Like your novel, the synopsis needs to flow together. Your paragraphs should not be a series of first this happens, then this happens, sort of descriptions. You do want to work in chronological order, though.
Don't forget relationships. Be sure to show some of the blossoming romance or impending conflict between characters, as well as the personal growth of your main character.
Your synopsis is also where an agent or editor can tell if your novel is well-plotted, so pacing is also important to keep in mind in the synopsis. If you can't make 2 pages entertaining, how can you make us care through an entire book?
I heard one author explain that the synopsis is typically like looking at your novel with a 1000-foot view. But every now and then you can zoom in with a line of dialog or a little detail that will really suck the reader in.
I'm sure this list isn't exhaustive and there are as many ways to write a great synopsis as there are to write a great novel. Have I forgotten anything major? Any little tricks you'd like to share for crafting a kick-butt synopsis?