Anyone who's randomly reading my blog and doesn't know Paul Graham should just stop now. His subject matter is both focussed and applicable to a wide variety of situations; his ideas are poignant; and his manner of presentation is perfectly tailored. From his "How to Convince Investors" piece, posted 2013.08 (emphasis mine):

The most important ingredient is formidable founders. Most investors decide in the first few minutes whether you seem like a winner or a loser, and once their opinion is set it's hard to change. Every startup has reasons both to invest and not to invest. If investors think you're a winner they focus on the former, and if not they focus on the latter.

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There is a role for ideas of course. They're fuel for the fire that starts with liking the founders. Once investors like you, you'll see them reaching for ideas: they'll be saying "yes, and you could also do x." (Whereas when they don't like you, they'll be saying "but what about x?")

But the foundation of convincing investors is to seem formidable, and since this isn't a word most people use in conversation much, I should explain what it means. A formidable person is one who seems like they'll get what they want, regardless of whatever obstacles are in the way. Formidable is close to confident, except that someone could be confident and mistaken. Formidable is roughly justifiably confident.

There are a handful of people who are really good at seeming formidable—some because they actually are very formidable and just let it show, and others because they are more or less con artists. But most founders, including many who will go on to start very successful companies, are not that good at seeming formidable the first time they try fundraising. What should they do?

What they should not do is try to imitate the swagger of more experienced founders. Investors are not always that good at judging technology, but they're good at judging confidence. If you try to act like something you're not, you'll just end up in an uncanny valley. You'll depart from sincere, but never arrive at convincing.