i) Keep time with the pedal. Loudly.
ii)Improvise a ground bass. (For added challenge, pretend your piano is a B-natural instrument instead of a C-concert)
iii)Mimic, with very exaggurated gestures, whatever the conductor is doing, behind his back.
iv)Repeatedly press keys silently, as if testing the action. Stand up and begin tapping the strings inside the piano.
v) Break out a triangle or some cymbals and hide in the back with the percussion.
vi) Repeatedly adjust tie/hair/glasses/shoes/suit/tuxedo/dress.
vii) Play arpeggios/octaves in the air above the keyboard. Swing your arms in large circles. Stretch your wrists/shoulders/neck.
viii) Take out your cell phone and make a call. Better yet, have someone call YOU seconds before the piano entry.
ix) Screw up your eyes and forehead and repeatedly play the opening bars over and over again in the air, as if trying to remember how the piano enters. Shake your head after each attempt.
x) Breathe in and make a head gesture, placing your hands suddenly on the keys, as if about to start in a horribly wrong spot.
xi) Stand up and begin lecturing the audience on the Brahms concerto you are about to perform (assuming you're playing Chopin). Repeatedly look at the conductor and orchestra in irritation.
xii) Look around the stage in bewilderment, consult your day-planner, and go and sit in the audience.
xiii) Gesture and whisper irritably to your page turner even though you don't have any music on the piano.
Er... actually, I'd recommend putting your hands in your lap, head perhaps bowed slightly (or looking at the conductor). Overall don't let the audience see that you're very nervous. Think positive thoughts. After all, if you are playing a Chopin concerto on stage with orchestra, you really should have the piece in very good condition. Use your kinesthetic sense - are you sitting properly? Tensing up your shoulders/neck? Hunching over? Holding your breath?