Prepare before you go onstage. What you do before you take the stage can affect your onstage presence. Take care of your voice by drinking only room temperature water. This will help keep your vocal cords from constricting, which can affect the way you perform your lines.
Memorize your lines. Stumbling for a line can affect your performance, as well as those on stage with you. Read your lines until they become second nature. This will allow you to deliver them with conviction when you perform, which in turn can improve your performance and convince the audience you are the character you're portraying.
Emphasize your lines and movements. Dramatic emphasis works best. You want the audience members in the back row to get as much from your performance as those in the front. A dramatic emphasis on spoken lines and action will help carry the performance throughout the theater.
Speak clearly and with volume. There will be microphones around the stage. Know where they are and where to stand so they pick up your voice best. If there are no microphones, avoid speaking with your back to the audience. Angle yourself so the sound carries.
Recover and carry on when a mistake is made. Hesitating or drawing attention to your mistake is bound to happen. It is how you recover that counts. If you mess up a line, continue it as best you can until you regain your place. If you trip and fall, get up and continue acting as if nothing has happened. If you recover from mistakes smoothly enough, the audience may not notice.