I think playing the piece and finding parts that are already automatic, then play the first bar of those sections and stop, because else you would do something that is already automatic... waste of time. Often every bar is part of an automatic section, you then only need an overview of the piece, and merge all the automatic sections to one whole automatic piece

Hofman says that we automaticly associate the music with a lot of things, some of them are part of the piano and the practice room, and this is subconcious. So if we play at one instrument only, we can have memory lapses when we try another instrument, and this is disastrous if we perform... So the remedy is to try the piece on a couple of different instruments, especially that one that we will perform on.
Also re-learning a piece several times makes it automatic.
To become less tense while performing I suggest this:
Learn some eastern philosophy, you can even try meditation or yoga if you want,
then FORBID yourself to think doubtful thoughts... just think about all the people that like your playing, if someone doesnt, then thats their problem, and probably they dont know what they are talking about

Also talk with people that encourage you and believe in you, dont listen to the others.
It is important not to care about what others think.
Playing as good on stage as in the practice room is a skill, and every skill can be trained. Make yourself more and more stable and notice how your performances improve, dont expect it to be allright in a few days

Stage fright isnt a permanent condition. I still have it, but much less...
And when you get nervous (I take it you get nervous) dont be afraid of that, start thinking about positive experiences, and think about times where you have been nervous, but when you started to play it got better, and everything turned out fine, so why shouldnt it this time...