You must know how to use your voice as efficiently as possible, with the least effort necessary. And you must know this BEFORE trying to increase your volume, so that singing loud is no more stressing to your instrument than singing soft.
You must increase your vocal stamina gradually and steadily. Never increase your vocal volume suddenly. You may injure it. If you haven't been singing full voice much, this becomes even more important. Once your stamina allows full voice without strain, sing full voice several days before a strong performance.
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