Remember to make an impression. Primary objective is to sell them your idea but a secondary one is to sell them yourself. You want to make sure they remember you so that if they don't follow through on your idea, it will be easier to submit there again in future. You want to go from being, "Here's a script sent in by Mr. Scriptwriter", to "Oh, here's a script from that Magnum guy that came in to see us a few weeks ago? Remember him?"
And by make an impression I don't mean play the fool or wear a hat, I mean the obvious stuff, be friendly, polite, charming. They have to know that you're an easy-going, respectable person that they can work with through the development process.
Regards clothing; you'll find the biz is very laid-back and casual. Unless you're going in to see some shyster at William Morris, you'll find most people in smart but casual attire so you should dress accordingly. I can't remember who said it but I once read, "You should dress like you are comfortable with money," meaning (I think) that although you're smart but casual, you enjoy looking good, you don't mind spending money on shoes or a watch and you won't freak out or skip town when they give you thousands for development.
Some people will say it's not that important, the pitch is what counts and to a degree that's true but during a pitch, producers are not just listening to your idea (what?!) they're also sizing you up, trying to work out what kind of person you are and all they can judge you on is by what you say and how you look. Take care of the how you look part so all you've got to worry about is what you say. What I'm getting at is; don't go in there looking like you put your clothes on with a shovel.
I think a good litmus test for any part of your pitch whether your appearance, your demeanour or the pitch itself, is the word 'professional.' Bring everything up to that standard and you'll be in a good position.
