When you practice, concentrate on how your whole body, especially your lower back, goes into playing phsyical patterns - not just single notes, but patterns. Then, concentrate on keeping your body active, and the lower back engaged, during the succession of those patterns (ie the whole piece).
When you get on stage and are nervous, usually what happens is the body freezes up, the hands either feel cold or start sweating (or both), the blood flow is impeded, the mind races ahead of the body, or perhaps behind. At these moments, you should concentrate on the activation of your entire body to play large pieces of the music, especially concentrating on how your lower back participates.
I think this has two important effects: one, it brings your attention to your core, which will regulate the nervous condition of the rest of your body; and two, it takes your mind out of a neurotic state and into a broader, calmer, state of being. If you practiced engaging your body on patterns and a succession of patterns, you can grasp that practice and hold onto it like a life vessel.
Walter Ramsey