Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
But it requires no less careful study to maintain than to acquire this facility. Theory once mastered is not forgotten, and the pen loses but little of its speed by disuse: but this promptitude and readiness for action can be maintained by practice only. The best form of exercise is to speak daily before an audience of several persons, who should, as far as possible, be selected from those whose judgement and good opinion we value, since it is rare for anyone to be sufficiently critical of himself. It is even better to speak alone than not at all.
There is yet another method of exercising this faculty: it consists in going over our subjects in their entirety in silent thought, although we must all the time formulate the words to ourselves: such practice is possible at any moment or place that finds us unoccupied, and is, in some respects, more useful than that which I have just mentioned;
for we are more careful about our composition than when we are actually speaking and in momentary fear of interrupting the continuous flow of our language. On the other hand, the first method is more valuable for certain purposes, as it gives strength to our voice, fluency to our tongue and vigour to our gesture; and the latter, as I have already remarked, [*](Ch. iii. 21.) in itself excites the orator and spurs him on, as he waves his hand or stamps his foot: he is, in fact, like the lion, that is said to lash himself to fury with his tail. But we must study always and everywhere.