One cause of yawning can certainly be that your body needs more oxygen, so key to solving the problem is getting enough breaths in. Some exercises make you want to take deep breaths before performing it. Like the vocal warm up exercises before singing play a crucial role.
But more generally about breathing properly while singing one needs to follow through on following factors:
- Stand up straight.
- Don’t raise your chest when breathing.
- Instead, breathe down into your stomach. Your lungs will expand downwards and you’ll have more control over the release of air as you sing.
You can train for bigger breaths but some people find it difficult to breathe properly. I know a couple of people who were asthmatic and had this problem, and they found swimming helped them learn to breathe more deeply and take deeper, more controlled breaths.
One of the first thing you could do is to train yourself to yawn : yawn and alternatively inhale and intonate at a standard voice level at the same time, opening the mouth as wide as possible.
Do that slowly, several times before starting to sing.
It will flex your face muscles in a good way and place your lower jaw. A little side benefit is that you will be less likely to yawn after this exercice. Our singing teacher used to make us do this and a little series of other warmups before every session.