a method of breathing that i have found effective, is short relaxed inhalations and long slow exhalations holding the exhale for bout 8 seconds.
the short slow exhales does 2 things it slows your heart rate down and it eliminates c02 from your blood which could be both good and bad.
Another thing which helps is taking a full gulp of air before heading down, fill the lungs at the lowest point(done with your diaphram muscle) then fill the upper. when you this part your shoulders move apart, then finally i fill the throat and neck part before spitting the snorkel and heading down.
Ive dove 65-70 feet all day constantly with plenty of air left, using this method. I guess everybody has different ways, i guess use what you are most comfortable with.