Every day, opera singers work on improving their breath control, because it's an essential element of vocal technique. They must be able to sing very complicated musical passages, to sustain long notes, and to project their voices without microphones. They work all the time on their abdominal muscles, particularly the diaphragm muscle which runs along the bottom of the rib cage.
1. Practice Correct Breathing Technique
Breathe deeply from your lower lungs - imagine a rubber ring around your waist (see pic)
Breathe in and try to push the ring outwards.
Breathe in through your nose and out through your nose and mouth.
Avoid raising your shoulders as you breathe in - keep them relaxed and level.
2. Exercise your breathing muscles
Fill your lungs with the biggest breath you can, and let it out as slowly as you can with a hissing sound.
Try it again and while you are hissing have your partner count how long you can make the hissing sound.
Now do it again making "aaaaah..." sound.
Which one is harder? If you were to practice this every day, you would build the muscles and be able to make sound for longer periods of time, as opera singers can.
During the performance - pay attention to which passages seem to require the most breath control.