Stand as tall as you can without tip-toeing. Keep your head looking straight forward. Maintaining this posture will help you to utilize your abdominal muscles to support your breath.
Place your hand just under your rib cage and inhale deeply. Exhale slowly as you contract your upper abdominal muscles. These muscles help to support breathing tone. If you are breathing correctly, you should be able to feel these muscles contract. You don't have to do this the whole time you are singing, but it helps to try this exercise to get an idea of what proper breathing for singing should feel like.
Open your mouth as wide as possible by dropping your jaw down in a relaxed position while tilting your chin in toward your chest. This will help you to utilize your vocal cords more effectively when singing higher pitches. It also gives you a greater area for resonance in your face and mouth.
Think about descending upon the note that you are about to sing from above. This mental trick helps many singers from singing higher pitches "flat," which means that they actually aren't singing high enough.
Warm up for five to 10 minutes before a longer vocal session, singing softly at first. Practice every day for about 20 minutes. You may want to build up to longer practice sessions (30 minutes to 1 hour) as you gain strength. Through warm-ups and regular vocal exercise, you will help to strengthen and maximize your vocal tone and ability for singing higher pitches.