part of it must be the instrument and for some, like me, familiarity with it. i've not often had the luxurious experience of a bosendorfer or steinway that is in new or good condition. but, once you have experienced that - even if you are a halfway good musician - you can sound a lot better than on the usual upright or wacked out grand in the practice room.
what i don't understand is how performing artists can sometimes play on an instrument they are completely unfamiliar with. of course, for others (hearsay from the steinway factory) they are in control of the exact piano, the exact touch, the exact tone and probably the exact tuning.
then, there is the 'zen' of really good performing artists - that for me is simply prayer. you know - i think everyone has a bad day once in a while. but there are those great moments that you play or hear someone else play really really well. your fingers are limber, they do exactly what you want, what you hear in your head comes out, and you feel sort of 'guided.' i've only felt this maybe twice in my life - and i think both was due to praying before playing. so, i call this the spiritual side.
then, there's the physical. exercise, diet, rest. maybe the other 'zen' is finding a balance in your life so you exude the most relaxation to your audience. if you are uptight, then the audience will be uptight (right?) andre watts may be the extreme (and perhaps afew others)... since he looks like he's sitting down to a cup of tea and from the neck up you just look at his head (i've done this at a recital of my piano teacher, too) and he looks like he's just admiring the scenery (unless it's a really difficult passage and there's sweat dripping down his nose).
perhaps learning to control your nerves is another aspect. some people smoke, some drink, but for me - it's a banana milkshake. the fight between adrenalin and breathing is a learned thing. if you get breathing too fast, it's not good. so you have to control your breathing. first to get yourself relaxed and then to control your nerves - and get into that performance mindset. the best thing i found was to reconcile in my head the possibility of a mistake and have the alternate plan for those unexpected jumps to the next few measures, bars, or page. once everything is settled in your mind, then you start. if you start too early or without thinking - i think it makes the pianist look bad and sound bad. who wants to hear a broken record when you get to that spot you worried about and can't get past it.