I think the most difficult lesson to learn about preaching (especially in the beginning) was prying myself away from carefully crafted notes. This quote, from maybe the best preacher of the past century, reminds us to study hard and then become sensitive towards supernatural editing of the Spirit in the pulpit.
Preaching should be always under the Spirit – His power and control – and you do not know what is going to happen. So always be free. It may sound contradictory to say, ‘prepare, and prepare carefully,’ and yet ‘be free.’ But there is no contradiction … You will find that the Spirit Who has helped you in your preparation may now help you, while you are speaking, in an entirely new way, and open things out to you which you had not seen while you were preparing your sermon.
– Martyn Lloyd-Jones, Preaching and Preachers (Zondervan: 1971), p. 85.