I think this is absolutely true, t-p.   In the theatre they're known as "tells" - something that tells the audience that the performer is uncomfortable with what he is doing, or thinking about something else.   An intentional "tell" can be very useful, but mostly they're unintentional, and detract from the performance.

One of Stanislavsky's central points (and it's worth pointing-out that Stanislavsky worked extensively with musicians as well as actors, and there is still an opera-house in Moscow named after him) is that you must never, ever, succumb to the temptation to think about "what you look like on the stage as viewed from the audience's seats",  because this will paralyse you with fear, and/or lead you to perform in a ridiculous way.  (There are several chapters in Stanislavsky's writings about the psychological ways to avoid falling into this trap).

Of course, "fear of not succeeding" remains in our minds as an undercurrent, but this is what Stanislavsky would call a "sverch-zadacha" ("an overriding imperative") - in other words it's an end result, and thus only achievable by the identification and application of a sequence of targets, or goals (or whatever term you like for an achievable and identifiable task) which are within our abilities if we have studied them and how to achieve them throughout our training.   A violinist doesn't go out onto the concert stage with the idea in their mind "I must succeed"...   that's already understood long ago, and in fact they have already succeeded by being offered the concert.   Instead there is a series of tasks and activities planned  ("I must watch the conductor carefully at bar 102", "I need to copy the oboist's phrasing in the second movement", "the ending must be senza vibrato as marked" etc) which careful and planned training tells him/her will result in the overall aim of a successful performance.  From the moment of taking up his/her position for the performance, even before the conductor has appeared, the audience "no longer exist" - the only thing that is important is the music immediately ahead.  Even the second movement doesn't exist - the job now is to play the first movement.

I've been meaning to contribute to this thread for a while - sorry it's taken so long, t-p!

There are a few things I am very conscious of to give me a better chance of playing well.  Getting enough sleep the night before is important, but hard if you are nervous about the concert.  Overcoming nerves is a big subject which I'll have a think about for later....

But as well as trying to get enough sleep, I yet to avoid food on concert day until after performance.  Snacks, like biscuits and mars bars, are fine, but a meal just makes me feel lethargic and makes my fingers stodgy.  The only time 3 found this useful was before a Brahms symphony, where it made me feel like a day Austrian in the VPO (I imagine....).  But otherwise, I just don't feel nimble enough to play.  I think you need to be hungry before playing.

Oddly, I never feel hungry when I'm in performance - perhaps there are too many other things to think about.  But I'm starving afterwards.

Knowing a piece too well from muscle memory can hinder performance.  On a couple of occasions, I've found myself thinking about cooking or whether I left the has on whilst my arms get on with playing a concerto.  That's fine, until your brain tries to take charge again...  Best is to learn a piece, then forget it, then learn it again.  Your brain remembers the relearning, and your muscles remember the first time round.  But that is very time consuming, as you have to allow years to forget it!

Tommo