Spurgeon once more: "If you yield yourself thus unreservedly to the mind and will of God, by-and-by, when you get out into the ministry, you will sometimes be impelled to use a strange expression or to offer an odd prayer, which at the time may have a queer look even to yourself; but it will be all explained to you afterwards, when someone comes to tell you that he never understood the truth until you put it that day in such an unusual way. You will be more likely to feel this influence if you are thoroughly prepared by study and prayer for your work in the pulpit, and I urge you always to make all due preparation, and even to write out in full what you think you ought to say; but not to go and deliver it memoriter, like a poll parrot repeating what it has been taught, for if you do that, you will certainly not be leaving yourself to the guidance of the Holy Spirit."

UPDATE
For more on this subject do see the debate that I had with David Wayne some while ago eg