Shaking in the leg is something I have experienced a lot when singing on stage (when my mind would whirl between hoping I don't loose my balance, hoping my long skirt effectively hides this unruly member and trying to will it to stop shaking - which usually happened the moment I hit a demanding passage and shifted my attention to that).  And I remember one piano exam where my arms and hands shook mightily while playing scales and arpeggios.  The poor examiner did his best to put me at ease ("get up, walk to the window, look at the trees and the birds, breathe...") and by the time I started on my pieces the nerves were under control. For some odd reason nerves don't get much to me when playing cello.

Any case, 1 1/2 years ago I took up voice lessons again, this time with an Alexander technique teacher.  The concepts of proper breathing, relaxation, flexible and supple posture, etc. have positively influenced both my singing and my cello playing.  Being aware of where I'm keeping the tension enables me to do something about it.  It's also a different way of thinking and approaching technique.  I grew up with a lot of do's and don'ts in music, some very rigid approaches and subsequently some solid technical abilities - which I couldn't effectively utilise, especially not on stage, because I was too focused on "getting it right".  Now that I let go of the tension of "it should" be this or that I get much better results and it's much more enjoyable to play and perform. 

For me Alexander Technique isn't so much a technique as a mental shift of focus. And this has helped me tremendously because I tend to get very tense when situations aren't ideal ...think churches, dead/too alive acoustics, voice feel scratchy/not in best shape, man at the sound board clearly doesn't know what he is doing, back-to-back performances, or tired from travel for two in a day in different locations - 300km to venue 1, 300km back home, eat and shower, 40km travel to venue 2, etc.  I had a lot of those kind of performances last year, yet I found I had confidence and that I could still relax, do the job to the best of my ability and let go of that which is out of my control. Shaky Leg only visited once.  Best of all, I enjoyed performing.

I love the Heifetz quote.  The first bit has been my motto for years - practice till you can perform it well even on an off day.  The second bit is the part I'm finally getting the hang of.  Let go!