Memorize all of your lines thoroughly. Begin with reading the script, song lyrics or other text completely at least once each night. Begin as soon as you have access to the text. When you are comfortable reciting the lines without the script, do so over and over again. Do not practice inflection, tone or emotion. Learn the words by rote so that you will not forget them if your own emotions or the actions of another performer or audience member distract you while you are performing.
Familiarize yourself with all physical aspects of your performance space. Know where the entrances and exits are, and how to get to emergency exits, alternative places to cross backstage, costume and prop lofts and bathrooms. It may be dark in these areas during your performance, and if you need to access them quickly you will want to know exactly where they are.
Rehearse your performance several times wearing your costume, including shoes and accessories. Work with the lighting and props you will use during the show as many times as you are able to before the performance.
Communicate with your audience. If you are a stand-up comedian, singer, musician or any other type of performer who breaks the fourth wall, or acknowledges the audience's presence during the show, this is as simple as being receptive to the audience's mood. If you sense that your work is not being well received, be prepared to switch to different material seamlessly. If you are performing from a script, be sensitive to audience laughter and applause, holding your performance briefly when it occurs. If you attempt to continue over the audience's responses, they might refrain from clapping or laughing out loud.
Speak loudly enough to be heard in the space. Practice this before the performance. Have a friend sit in different parts of the venue. Remember that a full theater does not have the same acoustics as an empty one. The presence of a full house will dampen your sound, so you will need to be slightly louder than you think. If your performance will be miked, or with microphones, practice with the microphone at least every day for a week before the performance. Be careful to avoid the unpleasant "popping" sound that can result from breathing hard onto the microphone. Remember, it doesn't matter if you give a good performance if the audience cannot comfortably hear you.
Take a deep breath, and relax. This is not easy to do for most people, especially right before a performance. Feeling afraid or apprehensive is normal. In fact, it is an old theater superstition that a lack of some degree of stage fright is a harbinger of a poor performance. Some techniques for relaxing before a show are meditation, heavy exercise, running lines (repeating the whole script by rote over and over again) and games. Experiment to find what works best for you, and don't be afraid to switch techniques.