Learn to use diaphragmatic breathing to help increase the amount of air you can bring into your lungs. Stand up straight, put both hands on your waist and breathe from your diaphragm. Correctly done, you will notice your sides expand as you breathe.
Practice inhaling and exhaling deeply by breathing in for four counts and breathing out for six. Completely expel all the air from your lungs by the sixth count. As you improve, aim to increase your breathing to eight and 10 counts.
Warm up before every performance with a buzzing exercise. Start on middle C and buzz up the scale a perfect fifth and then back to the original pitch. Move up a half step and continue the exercise. This will help increase your breath control and warm up your voice in a safe manner.
Sing a vocal extension exercise every time you practice. Start on the sound "hmm" and sing down a perfect fifth. For a soprano, begin on a D in the middle of the staff and continue ascending to the highest point in your range. Altos should start a third lower and tenors should start an octave below the soprano. The bass should begin an octave below the alto.