Gamasutra has an excellent article entitled : How to Give Your Game Idea Legs, which is basically a gathering of thought as supplied by a working panel from E3, on how to pitch your game idea to publishers...

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On the workshop panel were: Lee Jacobsen, Vice President of Business Development - Midway Games; Jeremy Gaffney, Vice President of Product Development ?¢_" NCsoft; Dan Kitchen, Vice President, Product Development - Majesco Entertainment; Randy Pitchford, President - Gearbox Software; and Timothy Campbell, Director of Business Development ?¢_" THQ. Moderating the panel was Trent Ward, a former veteran video game journalist now working as Creative Director for Backbone Entertainment's Vancouver studio.
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It is well worth reading.