Jack Miller in The Heart of a Servant Leader on preaching with power:
It seems to me that the fundamental issue in preaching with power and authority finally gets down to the power of the Spirit indwelling the preacher in a response to a total life of prayer, a holy walk in obedience, the keeping of a good conscience by faith, and a careful cultivation of moment-by-moment fellowship with Jesus.
...Basically I am convinced that men who do not make praying their first priority in life and ministry should not preach or pastor.
Miller makes five suggestions on preaching with power. Here are two:
Not only make prayer the top priority, but use your prayers to search out any unconscious areas of self-dependence, including the written sermon, self-preparation, training, official position, past successes or failures, the opinion of others, and your own self-evaluation as to your own appropriate style. Then shift all your attention away from these things to Christ...
Then make each sermon a daring proclamation of Christ, not just of the text, but of Christ in His glory and power.
Please save this latter. I want it for my own instruction and admonition. I need your prayers that everything in here will be fulfilled in me as well as you.