Pitches are the way most ideas are thrown about in today's business world. They can consist of a presentation (PowerPoint, handouts, etc.) or a simple declaration of intent. In the literary world (borrowed from the movie world), the best pitches are done in two senteces. Generally, this is all the time or space you're allowed.
The first sentence gives the main storyline, giving as much about the plot and characters in as few words as possible. The second sentence gives the style of the piece through a comparison to other works the listener (hopefully, an agent or editor) will know, to further the understanding of what type of work this pitch is. Let's start with the first sentence.
Above all else, the first sentence needs to be simple and concise. In general, avoid character names - unless they're really good or are important to the plot - in favor of character descriptions. Jane or Sam, for instance, become "the good girl gone bad" or "the lonely boy who all the girls love but is secretly a violent maniac". You get more character in your pitch this way.
Next, describe only the main plot arc. If you're writing a novel, the listeners assume there will be subplots, but for now, they're not important. Remember that books are bought these days because the editors believe the book can make money - and to convince their marketing people of that, the editors need to be able to tell them how to sell the book. They don't sell books with subplots; they do that with the central character arcs.
So, Jane's plot becomes "The good girl gone bad returns home to reconcile her new Los Angeles party girl image with her devout Jewish family" and Sam's plot becomes "The lonely boy who all the girls love but is secretly a violent maniac's world is destroyed when his past crimes come back to haunt the only girl who truly made him feel needed." Easy, right?
The second sentence adds to the plot with its comparison to other works. This is where you comment on your style. For example, if you're looking for a girl-power action story that has all the trappings of a romance novel, you want "It's Charlie's Angels meets Danielle Steel." We understand then, that's it's a comedy-romance-action hybrid, emphasis first probably on the comedy or action since you led with Charlie's Angels. If you say, "It's LOST meets the group dynamic of Cheers, with a dash of Xena", we're going to expect a sci-fi, mythology-intensive epic with a group of good friends that is reliant on a strong, powerful female lead.
Why do I use TV shows? Simply because more people are aware of popular TV shows than they are of popular books. I can say "Wuthering Heights meets Neil Gaiman" and many of you will think a possibly funny and dark sci-fi/fantasy meets epic but tortured romance, but if you don't know Neil Gaiman or Emily Bronte, you're lost. People are more likely to connect with the most well-known sources you can compare your book to.
Also, popular and successful comparisons are what grab the marketing team's attention - if these elements grabbed lots of people's attention before, then they can do so again. Also, it's often a good idea to use recent comparisons, because comparisons to Victorian novels don't mean anything to today's book market.
So that's it: a simple, straight-forward 2 sentence pitch. How do you know it works? Use it to describe your book to strangers or at least people removed from your writing circle. Watch their reactions to gage how good it is - is they ask specific follow-up questions, you're on the right track.