Watch a few minutes of an NFL football game. Count some people on the field and in the stadium. Remind yourself that none of them care about how well you do in your show, much less even know about it.
Source: performer in a production of Hello, Dolly.
On the way to the theater each night, count the number of cars driving the other way. Those are all people who have probably never thought of your show, likely never will think of your show and who are simply unconcerned with how well it goes, or whether it even “goes” at all.
Source: Fiora Contino, Opera Conductor.
Remind yourself what you are really doing: dressing up in clothes and pretending to be someone you are not while you make loud noises with your neck. In the orchestra pit are people poking on things, maybe blowing into things or hitting things to make noises at certain times. You happen to be coordinating your neck sounds with their noise.
Source: Soloist at the Metropolitan Operadownload PDF
You’ve had hours and hours of training in your craft and a lot of rehearsal for the production. Every bit of that is to get the fine-tuned motor skills, creative impulses and emotional reality to the point of being automatic. You’ve examined and explored the role from as many angles as you could, you pushed yourself as an artist and you have grown from stretching your bounds out further. At this point - the point of performance - there is nothing left to prepare. Trust that your preparation will hold, that it will get you through, and go enjoy yourself.
Source: Lieux Dressler, Acting Coach.
Focus on breathing deeply
In the wings, think about breathing in and breathing out. Count slowly as you breathe. Panic leads us to take short breaths: counteract that by paying attention to each breath and keeping your breathing slow and easy.
Source: performer in a production of Joseph and the Amazing Technicolor Dreamcoat.
Stay in the sequence of events
Since fear is largely fantasy, having a very specific sequence of events gives our minds something real and in the present on which to focus. Say something to yourself like “first I do this, then when this is done, I do this. When that’s finished i do this.”
Example: during a quick change. First I take off the jacket and set it down. Then I take off the pants and set them down. Then I un-botton the shirt and set it down. Next I pick up the pants and slide them on my right foot, then my left foot, then I pull them up and zip up the fly. Then I pick up the shirt and slide on on my left arm, then my right arm. Then I button the bottom button, then the next, then the next, etc.
Literally talk to yourself like this, almost like your narrating what you are doing. It helps keep you in the here- and-now.
Source: dresser in a production of 42 Street - a show with a bajillion quick costume changes.