Because stage fright usually disappears after a few minutes on stage, be sure you know exactly what you're going to say and do from the moment you step out in front of the audience, according to Psychology Today online. Whether you're in a play or giving a speech or presentation, rehearse your opening lines or remarks over and over again out loud to get used to the sound of your voice--and do this even while you're waiting to go on. If you have the confidence to get off to a good start, that should help calm your stage fright and help you deliver a great performance.
The performers who support Music Cleveland suggest deep breathing from the diaphragm to calm your nerves and relax your body. Usually, breathing in through the nose and out through the mouth is particularly helpful and relaxing. It's important to start those slow, deep breaths as soon as you start to feel anxious, but be careful not to hyperventilate. You should also continue to breathe once you hit the stage, because that can sometimes be the moment when you begin to freeze up.
Your audience, whether it's people attending a play or listening to your speech, wants you to succeed. They want to be entertained and/or informed, so your audience and fellow actors or presenters are all pulling for you. Finding a friendly face in the front couple of rows can help, too. Along those same lines, it's also helpful to recall positive experiences you have had on stage and with an audience.
In Psychology Today's "Fighting Stage Fright" article, the writer notes that acknowledging your anxiety and your specific fears (worrying about forgetting your lines, giving a bad performance, etc.) can be helpful. When you can clearly identify your fears, Lybi Ma writes that you can replace negative images in your head with positive ones and focus on them as you proceed.