I was preparing for a "Bardic 101" class last month. In and among 40-50 minutes of "what is a bardic circle?" and "how do they work?" and so forth, I came up with two bits of the Performance Gospel According to Teleri. Here goes.
1) Thou shalt perform that for which thou hast passion.
Or, "Do what you love." (This, is occurs to me, is another reason why we don't have much genuine medieval music in the SCA - not too many people have a passion for it.)
7/10s of your performance will be carried on your enthusiasm for what you're performing. Technique and style matter, but not as much as you think they do, particularly in an informal bardic circle setting. Whether you are crazy about the "St Crispin's Day" speech from Henry V or "The Scotsman," you should go with that. If it's not SCA-appropriate, you may be in the wrong venue - consider a local coffeehouse instead.
If you are in a group, or if you take requests, you may not love-love-LOVE every single piece in your repertoire. That's okay. But if you find that you really don't like most of what you're performing, or have no excitement for it, or that it doesn't stir you, you're performing the wrong stuff.
2) Thou shalt not apologize prior to performance, and only rarely shalt thou do so afterwards.
If you forget the lyrics, drop your bard book into the fire, drunkenly careen into a bystander, or belt out "Bend Over, Greek Sailor" at the Children's Bardic, you can apologize. If you do something genuinely rude, or if you can't finish the piece your audience was enjoying, you can apologize.
Prior to a performance, it's an attempt to manage expectations. You're telling them that you haven't practiced/aren't that good/are new/are sick/are drunk in the hopes that they won't criticize you too much. Newsflash: It doesn't work that way. They are there to have a good time. They want you to be good. Help them to believe that you will be good, by projecting enthusiasm and confidence, and they will believe that you are good. Even if you missed a stanza/went flat/were too quiet/were too loud.
Moreover: either you are good enough to perform, or you aren't. If you are, do it and don't apologize. If you aren't (because you haven't practiced or you have laryngitis or whatever), then don't. Don't tell us that you're awful and then expect us to listen anyway.
After a performance, an apology doesn't help anything. Chances are excellent that all those dozens of mistakes you made went entirely over the heads of your audience. Unless you came to a screeching halt partway through, they either didn't notice or have already forgotten. Calling their attention to it is counterproductive. Act like you meant to do that and they'll think that you did.
And a Golden Rule: Anything short is fine.
I have been to long performances that I have loved, so I would not make brevity a commandment. However, in general, if you are nervous or new or unsure about your bardic ability, do something short. Scadians are a warm and polite audience, particularly when their patience is not taxed. They will sit still for anything, provided it comes in under 3:30 or so. Yes, even the most uneven novice attempt at a shaggy dog story will be given polite attention for the first 3-5 minutes.
If you listen to people when they complain about bad performances at bardic circles, you will notice that the common theme is that something was not to their taste, and it was long. It went on and on and on, they moan. If you think you are losing or have lost an audience, wrap up your piece. Better to bow out early than be remembered as That Bard, the One Who Droned On.