Learning to improve your voice is a gradual process.The process will become automatic, but it takes daily practice and often several years of study, depending on the individual. There are varieties of voice exercises to strengthen and shape your voice. Here is a 15mins breath control exercise for sustaining:
Bend over at the hips, not waist, so that your spine is straight. Be sure not to lock your knees straight, but instead let them soften with a slight bend. Feel your face, arms and body relax. Be like a rag-doll. Inhale so that you feel an expansion low in your body, even into the thigh area and low back.