Good breath support is derived from established abdominal breathing techniques. While other breathing methods may allow for equal or greater amounts of breath capacity, they do not allow for the breathing muscles to regulate each breath during exhalation which is absolutely necessary during singing. If your breath is high in your chest or in your ribs, you may find that energy is wasted and tension is developed that may be transmitted to the neck and larynx, and you should work with a voice teacher to develop a lower, healthy technique.
Breath control is the relationship between your breath and vocal folds, and it largely determines how long a breath will last while singing. Although some people have much larger lung capacities than others, how much air you can physically hold is much less important than how you pace what breath you have. Like swimmers, singers have a psychological aspect to their breathing; if they can overcome the urge to breathe, they will realize they can make it further than they thought. A suggested exercise is blowing out imaginary candles, eight in a row in one breath with separate abdominal impulses, on an "f" sound. Do four repetitions with deep inhalations in between, each silent and relaxed.
Breath pressure is applied directly to your vocal folds in order to make phonation occur. This requires coordination to create beautiful singing and, more importantly, healthy singing. If application of breath pressure and the closing of the vocal folds aren't synchronized, a breathy or tense onset will occur, both of which can lead to voice disorders. For a healthy onset, try breathing in as if you're beginning to yawn. Feel your breath expand low in your body, and start the onset simply, without overt physical effort.
Among many details of proper posture necessary to ideal singing, the abdomen's placement is crucial to a superior breathing technique. The lower abdomen, from the waist down to the pelvis, should be felt to be held in rather than sagging forward, without exerting tension. The upper abdomen, from the waist up to the ribcage, should feel free atop the lower abdomen. This combination provides allowance for full, deep breathing as well as stability and control.