Stand as tall as possible with your shoulders level and your head facing straight forward. This type of posture will help you to maximize your air flow and muscle support.
Utilize your abdominal muscles. As you inhale before singing a phrase, you should feel your abdominal muscles expand. When you begin to exhale as you sing, imagine your abs contracting and lifting to support your tone. Place your hand on your belly for the first few times to get used to what this feels like.
Warm up gently and gradually. Trying to sing too forcefully without warming up can damage your vocal chords. Try humming gently or vocalizing on a syllable such as "la" or "ma." Sing a variety of notes or simple melodies, using a gentle voice and staying in a comfortable range for your voice. As you warm up, gradually add notes that are higher or lower than your normal range, and begin to build your power and volume as you go.
Practice the parts of a song that you want to sing loudly in a normal voice first. Get all the notes down before you add volume. It won't sound good if your powerful singing is inaccurate in pitch.
Belt out powerful parts of a song by imagining that you are projecting your voice toward the back wall of the room in which you are standing. If it is a small room, imagine that you are in a large concert hall.
Open your mouth by dropping your chin and tilting in inward to maximize the amount of space that you have available for resonation as you sing.