Do not try to speak too long without renewing your breath. Nature cares for this pretty well unconsciously in conversation, and she will do the same for you in platform speaking if you do not interfere with her premonitions.
A certain very successful speaker developed voice carrying power by running across country, practising his speeches as he went. The vigorous exercise forced him to take deep breaths, and developed lung power. A hard-fought basketball or tennis game is an efficient way of practising deep breathing.
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