Awhile back (err, 20+ yrs?), it was quite common to use the technique Don describes and I don't think it was new at that time. Some key points to remember -
Newer divers tend to breathe short and rapid - rather than long and slow. You don't want to take deep breaths (pulling more in your lungs than is normal/comfortable), but you do want to fully inhale *and* fully exhale. Once you can do this, try the following:
Breathe from your diaphram - there's plenty of online sources that tell you how to accomplish this.
For short changes in buoyancy, use your lungs. After you have accomplished daiphram breathing, you can breathe from the top 3rd to temporarily increase your buoyancy. I would think this is considered an advanced technique, but once you begin to get comfortable with "just diving", give it a try.
To bring this all home, here's what Tom Mount has to say:
Tom Mount, president of the International Association of Nitrox and Technical Divers, recommends doing a t-ai chi breath exercise: breathing in for six to eight seconds, a natural two or three seconds pause, then exhaling for six to eight seconds. "While you're doing it," Mount says, "concentrate on breathing with your diaphragm. This is the way we were born breathing. But we lose it through bad habits and our cultural upbringing. You go to school and they say, 'Chest out, shoulders up, stomach in,' that destroys diaphragmatic breathing." Mount suggests this exercise: Lie on your back, putting one hand on your chest and the other on your stomach. As you breathe, make sure your stomach rises and not your chest. "Once you learn to breathe with your diaphragm," says Mount, "practice it all the time until it becomes a reflex."
Diaphragmatic breathing is the only way to get air into the lower third of your lungs, which is where two-thirds of the blood supply is. This breath technique will increase the efficiency of your lungs. It will enhance your ability to metabolize oxygen. Breathe with your diaphragm - and you'll improve your performance and endurance in any sport.