Dear Sora,
that's an interesting thread.
1. I think the most important thing was already commented: breathing. There are endless way of focus on your breath, and the best advice is probably try a bunch of them to see what (and how does it) works.
2. To me, everything starts on backstage, with a long session of yoga and stretching. I like to have at least half an hour, alone, to focus on my breadth and my body. Btw, this is fundamental to me: I need to feel what is my body like in that particular day. Then, this is very personal, I have a moment of prayer and I'm done to go to the stage.
3. Well, about your specific question, I understand the moment after the last person stop clapping and the moment I actually start playing as a moment of balance. Backstage, I was by myself, absolutely peaceful and relaxed; then, the crowd, the light, the adrenalin. The point is: both states of mind are not proper to piano playing.
4. When you are too relaxed, your mind travels freely, normally far far away from what you have to do; on the other hand, a shot of adrenalin (because this is the effect of getting on stage) also puts you in a state of mind that is not that good. So, as I said, you must balance again and focusing on your breadth is a cool strategy.
5. I also like to think about people I know that really enjoy what I will play (friends, mom and dad, my wife), and imagine they are my audience (regardless where the actual audience are them or 500 strangers). That makes me very confortable to play.
6. A final strategy that I used as a defensive tool and become sort of a signature: I always begin with a long work, with a quite easy beggining, that sets me in the mood of the recital or - at least - of the first part. A favorite example is Mozart's c minor fantasia before the sonata, or before the c minor Schubert Impromptu. Another one, particularly effective, are Satie's Trois Gymnopedies before Debussy.
Oxygenated regards,
Jay.