Remember to pack everything the night before and double or even triple check everything!! If possible, put your stuff by the front door or in the car the night beofre so you don't forget anything.
Try to curl your hair the night before. Don't remove the curlers until about 5-10 minutes before you're about to go up on the stage. When not on stage, put your hair in a hairnet to keep the curls from sagging.
Go over your steps in your mind while on your way to the feis and when you're standing in line waiting for your turn. In the car, you may want to listen to your music and do the steps in your mind along with the music.
Don't worry if you mess up at a performance or competition. Chances are only the judge will know that you made a mistake. If you try to switch feet, or even worse, stop, you'll end up making things worse and embarassing yourself.
If you do make a mistake, try to repeat the mistake when you do the step on the other foot. This way, even though you've made a mistake, the step will look identical on both feet.
If the music stops, keep dancing until you are finished your dance or until you're told to stop.
Make sure you get to the venue in plenty of time and scout around for where the stages at which you will be competing are. And don't forget your number!
While waiting in line, make sure that you're standing with proper posture and foot position. AND DON'T TALK! While walking across the stage, try to look as confident as you can, not as though you're about to pee in your pants.
Try to make eye contact with the judge as often as you can.
Remember to bow/courtsey to the judge and the musicians. As you're walking across the stage, keep proper posture (ie. don't swing your arms).
Be prepared to lose.
Don't eat (a lot) right before a competition, but make sure you have SOMETHING to eat. Otherwise, you may faint due to overexertion and heat from dancing, especially if it's an outdoor feis.
If you lose a hair band, pin, whatever, while you're dancing, ignore it. If your shoe lace comes undone, stop. The judge will usually signal for you to stop anyway.
If your hips are more than a bit wider than your waist and you can't put your dress on over your head, try crossing your legs and then pulling the dress on.
Try to take up as much of the stage as you can when dancing.
If you decide to wear makeup, make sure you don't go overboard. Try to look your age, even if you're an adult dancer.