I picked up a volume titled Homiletics, by Karl Barth just because I was curious as to what his slant would be. Like many in my generation, I've been warned off Barth and it is understandable given some of his leanings. But it is also true that, in his day, he was a bit of a reformer. I can't vouch for everything he wrote, but this section from Homiletics is spot on.
In his chapter on Actual Preparation of the Sermon, Barth begins by describing a young seminary graduate who has finally been installed as a preacher in a local congregation. He at once is keen to preach all those wonderful theological truths and insights gained over years of tedious study and reflection. But, Barth goes on to say, at some point, " time will come when they (the first years of ministry) have exhausted this abundance. A time of drought and emptiness will set in which only too easily can discourage and frustrate them."
He goes on to suggest the only true remedy: "One thing alone we must do, namely, open our eyes and see the treasure that is spread out before us, and then gather it and draw from the unsearchable riches and pass them on to the congregation. The encouraging 'Do not worry' must strike the heart of the discouraged preacher, for the heavenly Father has made provision, and we have simply to be prepared to listen to his Word. Our own inspiration by which we swear in the beginning will leave us in the lurch sooner or later. Then the exposition of scripture must replace it. This alone will endure.
"The first step in preparing a sermon is thus to realize that we must seek the material for it exclusively in the Old Testament and the New. This alone is the material we must proclaim to the congregation, for as the community of Jesus Christ it is waiting for the food of holy scripture, and nothing else."
Well said Dr. Barth ... well said!
(taken from Karl Barth, Homiletics (Westminster/John Knox Press, Louisville, KY) 1991, pg 92,93