Rehearse what you are going to play until you cannot get it wrong. Do not be tempted suddenly to do (on stage) something you almost remember from 10 years ago. Make sure you are certain what key you do things in. Make a setlist with titles, keys, and capo positions. You may even want to practice the links and the jokes.

Unless, like Ian Bruce, you have a totally bombproof voice, by all means go to the late night sessions, but sing as little as possible and as quietly as possible. Stop 3 songs before you know you ought to. You need your voice for stage. If you are on stage at noon, get to bed before 3am. Stay sober(ish) until the last night or two. Do not chainsmoke unless you usually do.

Take Strepsils, Vocalzones, or fisherman's friend (to taste) in case you ignore or forget advice above.

Learn to play some of your usual songs a tone down, in case you ignore or forget advice above. In such a case, also carry emergency brandy in the hope of the resurrection of the dead (voice) but don't overdo it - just very light lubrication, and very very light singing from as soon as you can make a croak. Do not overwork a voice you are trying to resurrect, you will make it worse.

CHeck you flies before you go on stage. Be in tune before you go on stage. Unless you can tune better than Martin Carthy, carry a spare instrument for each tuning you intend to use. Still have a story or two to cover late "tweaking" of tuning. A variant of the one about Martin Carthy and the rabbit in the bar often works.

Stay off cider. 47 strong coffees will make you hop around like a monkey up a stick, and probably need a waz halfway through your set.

Carry at least one spare roll of lavatory paper - a bodhran case is good for this.

Keep an eye out for the organiser's unexpected signal that you are to do one more song (or mone more song than you expected).

Treat the soundman like a human being, and with courtesy, and (s)he may return the compliment. Pre-warn him/her if you have eccentric requirements e.g. three instruments with wholly different input characteristics, or a stereo guitar output.

If the opposite gender (or same, according to taste) is impressed, carry and use condoms. My daughter tells me this is common coutresy in today's world.

I've never been booked to play a festival - but "be prepared"!