Great advice! Though I might gently query whether anyone ever completely gets to the point where they're never nervous - I think all of us are to a degree, regardless of how long we've been doing it or how many times we've done it before. I did it a LOT as I was an actor for 20 years, and even after I switched careers my role put me in a position where I had to deliver talks and presentations on a regular basis. And I always, without fail, got butterflies in my stomach before walking out onto the stage.
Point 1 is key. You have to be prepared in order to be relaxed. If you're a seasoned performer you'll be familiar with the situation. But even then - and even more important if you're less experienced - preparation and practice is everything. If you find you have time for one more run-through of your piece or your topic or your key points before the 'live' performance, do it!
Top tip #1: Our psychological state triggers physiological changes in us. The opposite is also true - you can trigger a psychological state in yourself by taking physiological actions. So if you're very nervous before you hit the stage you can relax yourself by taking deep, slow breaths. That will slow your heartbeat, and after a short while you will actually feel less nervous!
Top tip #2: Always take a bottle of water on-stage with you or ask for a glass of water to be available. Nervousness and/or talking will give you a dry mouth. And trying to talk in a relaxed way with no saliva is horrid! Pause, take a sip of water, and you'll be fine.
Top tip #3: Try and look at the audience some of the time rather than staring at your notes or book for the duration. They'll love you more if you include them in the conversation. But don't try and take all of them in at once; pick out individual people around the room and talk as if you're speaking to them. It feels more personal for the audience and it's less intimidating for you, especially if they're smiling. Great radio presenters always say that they talk as if they're having a conversation with a single listener. In my experience, good public speakers do the same.
Finally, the single most important thing to keep in mind is Point 5... Always remember that no-one in that room wants you to fail. They're all on your side and willing you to succeed. They won't heckle. They wan't mutter at each other about you. They won't be thinking nasty things about you. And they'll be absolutely fine with slips and stumbles. In fact they'll usually like you more! Think of them as friends, not judges!
Anyone who's interested in seeing a shambling middle-aged man chatting to an audience for half an hour can see me in action on YouTube here. It's from a conference 7 years ago combining aspects of my acting years with my later career in a talk which actually may have some relevance to writing too, as it's really about creating believable characters!