C.H. Spurgeon, Lectures to My Students, Ch. 10.

If a man would speak without any present study, he must usually study much. This is a paradox perhaps, but its explanation lies upon the surface. If I am a miller, and I have a sack brought to my door, and am asked to fill that sack with good fine flour within the next five minutes, the only way in which I can do it, is by keeping the flour-bin of my mill always full, so that I can at once open the mouth of the sack, fill it, and deliver it. I do not happen to be grinding at that time, and so far the delivery is extemporary; but I have been grinding before, and so have the flour to serve out to the customer. So, brethren, you must have been grinding, or you will not have the flour. You will not be able to extemporise good thinking unless you have been in the habit of thinking and feeding your mind with abundant and nourishing food. Work hard at every available moment. Store your minds very richly, and then, like merchants with crowded warehouses, you will have goods ready for your customers, and having arranged your good things upon the shelves of your mind, you will be able to hand them down at any time without the laborious process of going to market, sorting, folding, and preparing.  I do not believe that any man can be successful in continuously maintaining the gift of extemporaneous speech, except by ordinarily using far more labour than is usual with those who write and commit their discourses to memory. Take it as a rule without exception, that to be able to overflow spontaneously you must be full.