Even in performance classes, where the audience is composed of fellow students, people often get really nervous. I do too, but my nerves rarely seem to become intolerable, thankfully. They finish playing and are really disappointed with how it went. If the piece wasn't too long, the prof tells them to play it (or a movement) again. It is then, without exception, much much better. A way somehow has to be found of playing it as the second time without going via the first rendition.

In general I find that working a piece up through different layers of performance (teacher alone--students--small concert--recital) helps a lot. If one goes in at the deep end the result can be very embarrassing (I've been there). There is also the fact that in such performance classes as above the atmosphere is supportive, or rather more supportive than that of the average public audience would be - the "comments from the floor" following the performance are all too often unbelievably bitchy.

Over the years I've found the most useful thing is not to fight the nerves, but let them be. If one needs to pace (sorry, Ian!) around then pace away; I know plenty of seasoned performers who have dreadful 'gastric' nerves (I'm thankful I've never experienced that), but the dry mouth, sweaty palms, tightened muscles etc are all best dealt with by (a) accepting that they are there and (b) plenty of deep breathing.

My worst performing experiences have been when I haven't felt nervous before and then I've been hit by it partway into a performance.

On the other hand, the best feeling is walking out onto the platform and feeling the tension disappear as you see a friendly (hopefully!) audience, and then taking advantage of the heightened awareness that all the adrenalin has given you.

Several things come to mind -

I always make sure I eat well before performing, no matter how nervous I'm feeling.  Nothing too rich, vinegary or spicy, and not a huge meal, but something nutritious that can provide slowly released energy.

Remember to breathe properly, and don't get dehydrated.

Make sure you walk on stage decisively and briskly, with your shoulders back, head high (but not TOO high!), and a nice smile.  And don't lose your good posture by hunching your shoulders when you bow.  If you train your body to look confident, the physical signals can leak into your emotional state.

Wear comfortable but suitably formal shoes.  (My first violin teacher told me never to wear rubber-soled shoes when performing.  She said it was something to do with resonance, but I think it may be about one's state of mind.)

Of course nothing can substitute for thorough preparation.  But no matter how meticulously you've thought, studied, practised, and analysed, remember that the audience is there to enjoy themselves - and so are you!  If you're having fun, they'll have fun with you - and vice versa.

Most of what we worry about is focused on getting things "wrong."  But a mistake is only that, is over as soon as it happens, and beating yourself up about it does no one any good.  Learning to let go in performance of what you'd jump on in the practise room is difficult, but may be the key to reducing anxiety before going on stage.  I'd much rather play - or hear! - a concert with some "errors", but where the spirit is warm, sincere, and always looking ahead to what can be done with the music next, than one where the performer grimly hammers everything out with accuracy.

It is possible to prepare for this shift.  Once you've got the notes under your fingers to some extent, try as often as you can to play the music without stopping - or at least in large chunks.  Play it differently every time.  If you make a mistake, try to remember where it was, but carry on playing as happily as you can.  If you're playing from memory and suffer a slip, try to improvise your way out of it.  Be willing to make a fool of yourself in the privacy of the practise room (or in front of the teddies), and you'll be much better equipped to deal with the worst case scenario on stage.

I really must take all this advice myself some day...   

I did want to contribute something here but I don't think I have anything helpful to say on the subject - it's been ages since I actually got nervous about going on stage, at least when it's to play music. Which is all very lucky for me (at least, as long as it lasts ) but it does mean I'm not exactly a fount of useful advice. 

I do remember a long time ago playing a piece by Member Barrett in Darmstadt. He was giving a seminar so I repeated the solo piece I'd played for the first time the evening before. Just before I started someone in the front row piped up 'anyone have a spare score?'. A score was found and my performance took place with Brian Ferneyhough and Harry Halbreich carefully perusing the dots a couple of feet away. I do remember thinking something to the effect of 'oh bugler'. Followed shortly by 'hang on, Ollie, you might not be completely in control of this little number yet but they're seeing it for the first time and you've been peering at it for weeks so it doesn't matter who they are, you do know it better than they do'. That at least was helpful...

especially with the air conditioning on full strength.

I have an involvement with a professional recital series, which I've mentioned on these boards before. I usually see the performers just before they go on and I'm always impressed by their composure. Perhaps the audiences here aren't very intimidating.

Last-minute crises seem to allay nerves rather than aggravate them. A cellist came to us from Africa via Germany via Hampstead via a broken-down car via a flood-beset railway system and eventually a lift from Telford, arriving at our place just 25 minutes before kick-off. His performance was superb. Another cellist (what is it with cellists? - are they the new violists?) arrived from London only to discover he'd forgotten to bring his music. He got a friend to fax it to him with about 20 minutes to go. It didn't affect the performance at all.

Hermita, really??