Some singers tend to sing every note flat -- that is, about a half-pitch too low. Others only sing their high notes flat. Proper breath control is essential to stop singing flat and to reach the high notes on key. You will be amazed at the lovely sounds that come from your throat when you breathe correctly and hold your body in the right way.
Breathe from your belly. Allow your stomach to expand when you inhale, increasing the amount of air you take in. When you breathe out, push your diaphragm up to force the air past your vocal cords. Practice this breathing without singing first.
Stand while singing. Keep your feet shoulder width apart and keep your shoulders relaxed. Correct posture allows for proper breath control, which is vital to singing on key and reaching high notes.
Relax your neck muscles while trying to sing the high notes. Straining will make the note go off-key. Lift your chin slightly and smile.
Keep your tongue as flat as possible while singing and keep your jaw loose. This will help to open up the throat and allow for the free flow of breath.
Sing softly while you practice reaching the right pitch. Loudness or volume can come later. It is easier to control your breathing if you are not belting out the notes.
Practice with a digital voice recorder. Play the recordings back frequently as you work to reinforce your improved technique. In time you will be able to both hear and feel the correct pitch as you sing.
Think up. If you tend to sing flat, think up as you sing.