To answer that, you've got to learn to read like a writer. (Or, in this case, like an orator.) You can study these speeches that impress you to find the answer to that question and to improve your own speeches. Don't just listen to them and be impressed, but analyse them. Take them apart. What are the orators saying? How are they saying it? How can you do that in your own speeches? Here is a very simple example to illustrate what I mean.
In my church, we don't have a pastor who preaches every week. Instead, members of the church are asked to talk on various topics each week. This gives us a lot of practice giving speeches, but most of the speeches (or "talks", we call them) aren't very good. This is to be expected, because we're just ordinary people, and we haven't all studied how to be a good speaker.
However, twice a year, we have the opportunity to listen to the leaders of our church speak to us. These people travel the world, speaking to all the members of the church, and spend a lot of time learning how to be good speakers. Some of them are particularly powerful speakers that draw more than a usual amount of attention. I paid attention to how these people give their speeches to find out what I can do to make my talks more powerful when I'm asked to give one in church.
- They always look directly at the audience. They're not looking down at a piece of paper to remind them what to say. Obama has his teleprompter, but I don't have that, so in order to mimic this I need to memorize my talks beforehand.
- They always speak firmly and clearly, with a strong, loud voice. Some of the older speakers have trouble as their health deteriorates, but the best speakers are ones where you never have to wonder what they just said. Many of the ordinary church members mumble, or tend to slur their words together, especially if they're nervous. This takes away from the power of their message.
- They don't rush. They take their time. They pause frequently, and allow the audience time to absorb their words. It's easy to rush through what you have to say so you can get it over with quickly, but this prevents your audience from really understanding what you have to say.
- They get straight to the point. Most of the talks at an ordinary church service begin with "Hello, my name is XYZ, and I was asked to give this talk..." Half of them ramble on past that point, saying things like "I don't want to give a talk but I will anyway", and even when they do get around to actually talking they'll begin with something trite or overdone, like "the dictionary definition of TOPIC is...". None of the conference speakers do that. They assume you know their name (because it's written on the program) and that you want to hear what they have to say. They just jump right into their topic, usually opening with a story or other engaging remark.
As far as content, since I'm using religious material, I'm not sure how much of it will be applicable to you. But you'll notice that the very best speakers don't just cover the basics. They assume you have a basic understanding of the subject material and go beyond that, to expound on something new or even just to reiterate what you already knew in a new and different way. And you'd be surprised how much of a difference taking the steps I outlined above would make. Even if the speech you're delivering isn't earth-shattering-ly awesome, delivering it in a confident, persuasive manner will make a world of difference.
Like I said, this is basic stuff. I can't help you with the more complex stuff. To do that, you'll have to do what I said in the first paragraph: read like an orator.