The perfect pitch is just as important as the perfect first sentence, paragraph, or chapter. Without the right pitch, your manuscript could sail off into the outfield despite how brilliant it might be. Effectively communicating what your book is about, and why it's brilliant, is the key to getting an agent or editor interested enough to read beyond the query, or ask for pages after the pitch session.
In a query letter, you should almost always start with a sentence that says the genre, word length, and why the manuscript is right for that agent/editor (this second part shows you did your research on them and helps personalize it). Some like to put this at the end but the majority of agents that I've talked to, worked with, or read their blogs like this at the beginning. The next part is the one-sentence hook. This is basically a summary of your book in one sentence. This does not mean you need to cram it all into a long run on sentence, the opposite in fact. It needs to be under 35 words or so (Twitter length is a good rule of thumb), highlight what is most interesting about your novel, and make them want more.
Following that are two to three short paragraphs highlighting the remaining elements in your novel that are interesting/important. Please remember, this isn't a grocery list, make it flow and read interesting. Think of the voice of your book and try to capture that a bit when you write this part.
Sum it up with a short sentence thanking them for their time and giving them multiple ways to contact you. Want to try those pitches out? Tonight on the #WritersRoad chat at 6:00pm PT on Twitter (via Tweetdeck is easiest) we're chatting all about it. Come join us!