To a degree it depends on the performance, but for ones that really, really feel "important" to me, this is what I do :
I will need some time alone.
I will have "sufficiently eaten" and will have juices and water close at hand. I will have dressed knowing in each moment while I am dressing that I am putting on some sort of official uniform in preparation.
I get very quiet and work to find an intense and focused calm inside of me and all around me. I summon every fiber of myself within those moments. I work to clear my mind and just be completely present in each moment. I work to be very alert to what kind of thinking may be trying to take place within me, and at all costs I will reach deeper within myself to find that part of me who knows I can walk on stage and deliver.
I will have a journal close at hand which I am both reading from and writing in. During this time, if I am focused properly, I am often recieving deep inspirations and insights about Life, Music and my individual purpose.
As the time draws closer, I enjoy having a person who is dear to me and whom I can deeply trust, with me during the last 10-20 mins before I go on stage. Either to talk with, or just be with.
I will have all of my music scores back stage, and as the time nears for me to go on stage, I will set up a music stand backstage where I will place my music score, open to the piece I will be playing.
Then, out I go.