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Don't get bogged down in minor details. I've seen so many pitches where they spend time extolling the virtues of a revolutionary AI pathing system, or a new whiz-bang environmental destruction simulation model, despite the fact that those features have almost nothing to do with the core gameplay mechanics.
If it's a minor part of the experience, your publisher will assume you're going to come up with realistic AI behavior and a new physics system during the course of development anyway. If it's not, then you should ask yourself if it belongs in the pitch at all -- or if you need to start again and make it a focus of your new development.
Bring everything and ensure contingencies. Just because they're a big fancy publisher with more lawyers on staff than people in your entire team, don't assume they have everything they need for you to do your pitch.
If you have a demo or video, take a powerful laptop with a big screen, long lasting battery, and everything saved to it. Pocket wall projectors are getting cheaper by the day, and you'll kick yourself for not having one when the local one has died or doesn't work with your file for some reason.
Take printed copies of your written material with you (this is important later, too). Make sure your co-presenters have a copy of everything too, in case of hard drive corruption. Assume you will have no internet access to get another copy from home.
Save video files in nearly every format you can think of, including a smaller version for your cell phone. In the worst case scenario, you can then at least hand someone your phone and let them watch your trailer on that. It's better than letting your presentation be completely ruined by unforeseen technical hiccups.
Practice. Most pitches take around five to 15 minutes, depending on how interested the publisher is, your reputation as a developer, and how busy the time of year is. Typically you'll be lucky to have five minutes of quality time at something like E3, so it's critical to have your presentation down pat and within a small time frame. If you're given extra time, expand on key points but make sure you cover everything in the minimum amount of time.
Rehearse your presentation over and over again to anyone that you can get to sit down and listen. Note when they start to look bored – that's a sure sign that you're retreading old ground and you need to switch to a new topic before that point. If you have a gameplay prototype, practice playing it in time with you or your co-presenter's dialogue to ensure you're demonstrating the features they're currently talking about. (Bonus points if you're both playing a multiplayer game and you include some scripted smack talk).
Remember the key points from your college debate class -- explain what you're going to be saying, say it, then tell them what you said. Present your main arguments for your game (i.e., the unique selling points) up front and return to them throughout your time talking.
Face time is critical. Yes, we have email and Skype and all sorts of wonderful technological bridge-builders that allow us to communicate across distance or time barriers, but nothing matches the immediacy and value of being in the same room as the people you're meeting with.
For one thing, you're guaranteed far more of their attention, and you're less likely to be interrupted as their staff knows they're in a meeting. There's also always more opportunity to discuss your ideas afterwards in a face-to-face situation.
This, more than the chaos and loot on the show floor, is why E3 and GDC are important events. So, dress well. Get that haircut you've been putting off for the summer. Have business cards ready to hand out when asked for one. Greet everyone with a handshake and a smile. Make small talk about the convention / flight / rental car mixup / local sports team.
Read How To Win Friends And Influence People, it's probably the greatest book ever made on how to be a nice person in a social or business situation -- and you really want the publisher to think you're a nice person at the start of the meeting.
Confidence. I get that you're nervous. There's a lot of money and time invested in this pitch and you want to get it right. However you have to put that out of your mind and act like you've already won the day. You might not believe it, but the people you're meeting with actually want to see you succeed more than anything. It's their job to find a developer to make their game, and they're rooting for you to be the one. If you're confident in your game, they'll be confident in your ability to make it a success and, in turn, that their job has just been made a lot easier.
So with that in mind you're already walking into a receptive room. Smile and look happy. Think about what you're going to say before you say it -- a small moment's silence while you collect your thoughts is always more preferable to a protracted "Ummm", stammering, or worse, changing your mind and contradicting yourself.