Practice this exercise to ensure you are breathing properly: Lie flat on your back, and put your hands on your waist with your fingers facing the middle of your stomach. Take a long, slow, deep breath, filling up your stomach from the bottom to the top. Your hands should rise upward and outward as you inhale; you should feel your sides and back expanding as well. When you have drawn in a full breath, exhale slowly over a count of five. Repeat this 10 more times. Do this exercise each morning and night, and two more times during the day. This will improve your vocal stamina and power, as well as help you to breath more deeply.
Practice ear training. Using the piano that is perfectly tuned, or a keyboard, play a "C" note in your range and listen carefully. Play it again, and sing along to the note. If you are not perfectly in tune, repeat this sequence until you have mastered the pitch. Repeat this exercise with every note until you sing each note in the octave perfectly in tune with the piano.
Play a C major chord (C-E-G). Play each note separately, but hold the notes to form the chord. Listen carefully to each note. Play the chord again, singing along to the middle note, E. Hold the pitch until you are in perfect tune with the chord. Do it again, singing along with the top note, G. Repeat this exercise with a variety of major and minor chords. This will improve your ability to harmonize and sing notes within chords and scales, not just the root note.
When you can do this comfortably for several chords, do the same thing an octave below or above middle C to stretch and improve your range.
Practice good posture. Keep your chin level, your eyes forward, your head up and your knees slightly bent, never locked. Keep your shoulders relaxed. Do not slouch or puff your shoulders and chest out. Point your toes forward and relax your back and abdomen. Smile, then begin to sing.
Practice singing exercises and enunciation. Sing the words, "doe, ray, mi, fa, so, la, ti, doe" in every major and minor scale. Use the piano to keep pitch as needed. As you sing, exaggerate your mouth shapes with each vowel. Make significantly different shapes for the vowel sounds O, A, E and Ah. Sing a song with lyrics, focusing on your enunciation. This will improve your vocal clarity and pitch.
Take vocal lessons. Seek out a teach who specializes in your vocal profession, whether it is rock 'n' roll, dramatic music, choral singing or another field. Work with your teacher on additional exercises to improve your vocal range. Ask the teacher for warm-up exercises to do before each performance or rehearsal.