I get asked this question quite a lot, particularly from newer players/performers and, oddly enough, have been contemplating organising at least one workshop locally (Sussex) for those who would like help. However, in the meantime...

Stallion's comment about being well-rehearsed is one of the keys to a confident performance. In my experience, it can take several weeks, if not months, to get a song/tune down to the point where you can perform it with no worries. Many people learn the words/notes/chords to the point where they think they do have it down pat - but, actually, they don't. Once the mechanics of getting the structure into your head are done, and you've learned it, it's important to allow a period of rest - say a couple of days - and then see if you can perform it straight off, to a mirror, to a partner, or to a small group, without a single mistake. You may or may not be able to do that. When I'm learning a new song I sing it over and over and over again - in the car, on the bog, in the shower, walking along the street (yes, I was that loony...), everywhere I go. If it's a guitar or other instrumental piece, I'll play it constantly, over and over again, without a break, for day after day - so that it becomes second nature. When it becomes second nature, you can concentrate more on performance and presentation than the worry of knowing the piece. I never use music stands, and I never take books or music or set lists on stage. I make sure they're embedded in my head.

I've been performing quite a long time now, and rarely get nerves, but I still always start a set or a performance with some idle, silly remark or chit-chat - nothing too long or complicated - just to break the ice. A burst of audience laughter relaxes them as well as you and gets your adrenalin flowing. As to quiet periods before a performance, that may also suit you. If I'm doing a spot in a club where there has been joining-in singing, then I make sure I join in too - gets the vocal chords going and some energy generated. I was doing the opening half-hour for a concert in a theatre some weeks ago, and walked in from the wings as the lights dimmed and the audience chatter died. As I came on stage, there was a large round of applause (and ironic cheering from the poor fools in the audience who knew me), so I just sat straight down on the chair with no words and played the first instrumental. That's another option

Having said all that, even hardened professionals can get stage fright. The important thing, should this happen, is to have some pre-worked out coping mechanism or strategy to click in. A joke, a re-start, a change of number...

There's loads more that could be said, but these are just a few tips from me. Others will almost certainly add more!