In his book, Design for Preaching (1958), Henry Grady Davis offers some sage advice for young preachers (and old ones too!). Here are some of his nuggets.
The preacher has only so much time for preparation, and it is never enough time. If he devotes too much time to form and too little to content, if he studies the use of form as something apart from content, he may become a rhetorician. A greater danger is that the preacher will be concerned only about the content of his preaching, will never in his life study form enough to begin to master it, and will never become the preacher he has it in him to be.
Most students for the ministry, possibly with their heads in the clouds, want to learn an easy way to preach. They are not willing to put forth a tenth of the effort required for excellence in comparable fields. They can give sixteen reasons, some of them being theological reasons, why they should not be bothered about form. The minister must preach out of a busy life, full of hard work. He will not have time to bring every sermon to the perfection of a published poem or short story. But if he lets that fact excuse him for poor work, he is doomed.
Therefore, the apprentice preacher should study his work in three directions.
1. He should read good books on writing and on speaking.
2. The beginner should study the sermons of other men, the great ones of an older day, and all he can hear and read in the present. There is no substitute for the study, analysis, and evaluation of actual sermons, lots of them. Moreover, the young craftsman should study sermons not only for their thought and vision, but especially for their craft.
3. Finally, a man must learn the craft by hard and unremitting practice in his own work. That is why I hope you will from the beginning remember that, while it may be more, preaching can never be less than a medium for the highest work you have it in you to do—you or any man. Its possibilities are no more fulfilled in the customary unskillful puttering than the possibilities of poetry and music are fulfilled in the sung commercial. If you will bring yourself to think this way about it, you will have taken the first important step in the right direction.
I will tell you a professional secret. If you can find out exactly what the text says to you, and if you do nothing but say clearly and cogently what the text says, the sermon will get undeserved credit for originality and freshness.