Breath properly when you sing. Breathing helps to relax the body and will ensure that you have the proper breath support to sustain your voice. Breathe with the diaphragm when singing by using the muscles around your stomach to pull air into the pit of your stomach. To get the feel for breathing correctly, try yawning. When you yawn, your stomach naturally expands, and you fill your lungs with air in a natural way. You should try and mimic this experience when breathing in through your nose. Avoid breathing in through your mouth.
Practice singing in front of the mirror so that you have a better concept of what you look like, and will be less inclined to worry about whether you appear professional to the audience. Use good posture and check yourself in the mirror. Your head should be high, shoulders relaxed and tension should be released in the jaw, neck and throat. Tension will make it more difficult for your vocal cords to vibrate and create additional stress. Before walking out on stage, flex all of the muscles in your body to release any built-up tension.
Concentrate on the music and look over the heads of the audience or focus your eyes on the foreheads of the audience members. Prevent eye contact to help remove yourself from the audience as much as possible so that you can focus on singing. A little distance will look to the audience like you are moved by the music and not that you are ignoring them. In a large concert hall, you may not even be able to see your audience. However, pretend that they are sitting in front of you and sing approximately to the audience. If your audience is small and if you notice that someone in the audience seems entranced by your performance, you might find your anxiety greatly reduced if you direct your eye contact on that person.
Sing first in front of people you are comfortable with. You may even wish to start with an animal. Then gradually increase the number of people that you sing for. When you get to your first concert hall, sing in the hall before the performance so that you are comfortable with the surroundings and not surprised by the way your voice sounds in the new hall. Concert halls have a way of making our voices seem small and weak. However, the audience will perceive your voice to be much larger than you believe it to be. Sing as you do in your practice sessions and you will be fine.