Make sure that your instrument is in good working order.
Make sure that you have picks, batteries, capos, spare strings, everything that you need.
Own your own mikes and DI. Audition several before choosing.
Be sure that contracts, technical rider, and stage plot were sent.
Practice tuning your guitar. Use a tuner if necessary, but learn to be quick and accurate.
Learn how to properly adjust a microphone stand. Don't just manhandle them.
If you have problems talking, practice your stories and song intros also.
Before the show
Don't eat right before you go on, it bogs you down.
The best liquid is water, ice may tighten your vocal chords. Have some onstage.
Tune all your instruments before you go on stage
Get our your picks, batteries, capos, spare strings, everything that you need.
Meet and greet the sound person. Tell them how want to sound.
Be sure they understand your stage plot
If you can, give them an adequately annoted set list.
Talk with them about special effect/level needs you may require during the performance.
Do an adequate and complete
sound check.
Be sure they know your loudest levels.
Warm up your voice and hands before you begin your set.
Get psyched and focused before the show. Don't rush on before you are ready.
Budget your time. Be prepared to go on on time.
Going On
Have someone introduce you. Let them try the intro first on you.
Be sure that house lights are adjusted before you go up.
If Emcee can't adjust the mike for you, do it quickly as you come on stage.
The First Song
Should be something that you are supremely comfortable with.
Should set a tone for the evening.
During the performance
Make good eye contact. (Even if you can't see the audience) This is very important.
Be confident. Don't castigate yourself for mistakes.
Don't rob your songs of surprise factor by tipping the payoff in the introduction.
Change keys regularly, don't play song after song in the same key.
DYNAMICS - volumetric, tempo, and mood changes enhance the performance.
Don't be afraid of strategic covers. (But beware the trite ones.)
Taking breaks
Why do we take breaks? Respect the audience's time.
Break just long enough to sell CDs and clear the bathroom lines. Don't dawdle.
Retune and reprep anything you need before you go back on.
When things go wrong
Don't argue with the sound person from the stage. That's so lame.
Play through or stop and fix it. Be decisive.
Avoid dead air, but don't babble unless you have something to say.
Be prepared to break a string or three with a story or acappella song as filler.
After the show
Most audiences expect a meet and greet. It's better not to disappoint them.
If family and friends are out, greet the general audience members first.
Try to have a helper sell the CDs so that you can schmooze.
It's useful to have someone else break shrinkwrap on CDs, saves time.
When you are the opening act
Do NOT play longer than your alloted time. Ever.
Play your best material.
The best material for an opener may not be the same as for a full show.
Showcase the elements that make you special.
Other tips
There is something to learn from every performer, even things not to do. Watch and learn.
Watching the opening act tells you a lot about the audience and provides set ups.
Tape yourself. Audio is good, video is better. Learn from your mistakes.
Some people love set lists, others hate them. Find out which you are.
Some performers write set lists with fixed openers, enders, etc but include elective blocks.