This video covers both a beginning and advanced variant of a singing breathing exercise which I call The Balloon Squat. Not only will it strengthen your diaphragm, but it will also build your core muscles and improve your body alignment! Best of all, you can do it anywhere, and it only takes a few minutes to complete.
To learn more about breathing for singing, click here.
After practicing this exercise daily for a couple of weeks, you should notice more efficient breathing along with greater flexibility in your voice.
Note: Instead of blowing up the same balloon 10 times, you could alternatively blow up 10 separate balloons.