Outline your idea. Pitching is like performing a one man one act play. You must be a great story teller, totally compelling and never let the listener be ahead of where your story is going. So this is why it is imperative to outline your story. A pitch should be no more than 20 minutes long. Seriously. Don't think 30 minutes is cool. Unless you're Spielberg, it's not. So, outline 20 beats of your story. These are the progressions your character makes throughout the film. An example would be "We meet John and Cindy, they're a cheeky couple who..." That would be your first beat. "Then one day John comes home, it's eerie, mysterious, the house is empty, and she's DEAD!" This would be another beat. Your story should have about twenty of these, give or take.
Break the acts. When you pitch a movie, it's a great idea to let the listener know where the end of Act 1 and Act 2 are. This will clue them into the pace of the story. A general rule of thumb is Act 1 ends when the major problem of the film presents itself. Act 2 ends when the hero has set out to fix the problem, struggled, then figured it out until...an even bigger bomb drops, taking your hero lower or as low as he was at the end of Act 1. Then of course the end of the film is when the character uses his knowledge that he's gained throughout the film to ultimately solve the problem and put his life back in order. Again this is just general structure. But looking at your outline, break the film into the three appropriate acts.
Follow the character. Make sure that in every step of the way you are with the protagonist. You should be rooting for whatever he is fighting for and following his journey. If you switch points of view, be sure that the character you switch to has their own unique point of view. By following the "character" of your protagonist, you will invest the listener in the story. Go back, look over your beats and make sure the protagonist is striving to reach his goal (succeeding or failing) every step of the way. Also, huge note, be sure that the character is always doing the smartest possible thing to attain his goal in every moment. If the bomb is about to go off, but you haven't tied up the loose end from when he shagged the babysitter, don't have him stop and make a phone call. Diffuse the bomb first. You get the idea.
Be engaging. There's nothing worse than someone who is pitching with a lack of energy. That's not to say you should be hyper, but intently focused on telling your story with passion. If it's funny, let that come across (comedy pitches are a bit like doing stand-up.) If it's a drama, intensity is key.
Practice, practice, practice. This cannot be stressed enough. Once you have the story, this is where the real work comes in. While the pitch should not be memorized, to avoid sounding like a robot you should be able to pitch the entire story without looking at any notes. This will take, literally, several days of up at dawn, burning the midnight oil hard work on your part, where you say the pitch over and over and over until the point that it is second nature, which of course is the whole point! Be sure to practice in front of people. This will get you used to listeners and also you are then able to ask them what they got, didn't get, understood, or didn't understand.
Relax and have fun! You've done the hard work, you know you're story, now just sit back and tell your tale. The executive you're pitching to has heard many pitches. They know how to make the room comfortable. And if you mess up, just take a second to backtrack. Congrats, you are well on your way to pitching a movie!