Nose Breathing to Replenish the Lungs and Relax the Vocal Tract
The logical thing to do when preparing to sing a long phrase is to tank up and get as much air as you can so that you will have enough to complete the phrase. But, if you’ve tried it much, you understand that this is counter-productive. When the lungs become overcrowded, it automatically produces a faster rate of exhalation. This not only causes you to lose your air more quickly but also counters any attempt to pace the exhalation, turning as much air into tone as possible by the vibrating larynx. Your intake of air should be to satisfy the lungs, not fill them up.
Another issue with tanking up is that it is more likely a noisy inhalation, which usually means bad form and a displacement of your chest, which, as you well know by now, should remain relatively steady with little to no displacement during intake and exhalation alike. If noise accompanies inhalation, it means that the vocal tract is experiencing tension, whereas it should be relaxed.
Click here to get some useful vocal exercises
If this is a particular habit of yours, there are a couple of ways to correct it, besides just being aware of it and stopping. The first is onset exercises. Take the /s/ sibilant and do short, quiet staccato bursts and replenish the air quietly. You can do the same with /f/.
Another way is to breathe in quietly through your nose, which will slows down the ability to bring the lungs to full capacity and helps teach intake pacing. This is good too because is helps the vocal tract to relax by lowering the body of the tongue as well as the larynx. This puts the vocal tract, including the muscles in the abdominal wall in ideal position for singing. And breathing through the nose reduces the possibility of chest displacement.
Once you get a feel for the breathing system that nose breathing creates, you can transfer this technique to mouth breathing, which is the primary goal. And this kind of breath can be taken through the mouth not only slowly and paced but also quickly, when necessary, but always quietly.
Give yourself some time and grace to work through this breath management principle. The main thing is the silent intake of air. This almost always represents a relaxed vocal tract, which is always that goal.
Hope this helps!