Many other blogger’s recommend The Supremacy of God in Preaching by John Piper. I am almost finished with it, but Piper notes five steps that he takes to make sure that he is preaching not in his own strength, but in the strength that God supplies. He uses the acrostic APTAT.
Lest you think that Piper’s list is like some sort of magical formula or incantation to guarantee the presence of God in his words, my observation is that it is mostly self focused. It is getting oneself in a right position to be used by God. It is not that "If I pray these five things, then God will automatically bless my preaching." But in submitting oneself in this way, one is better open to be used by God.
The five elements are:
A-Admit to the Lord my utter helplessness without him. (John 15:5)
P-Pray for help (insight, power, humility, love, memory and freedom).
T-Trust. "Not merely in a general way in God’s goodness, but in a specific promise in which I can bank my hope for that hour."
A-Act "in the confidence that God will fulfill his Word."
T-Thank God. "Gratitude to God that he has sustained me and that the truth of his Word and the purchase of his cross have been preached in some measure in the power of his Spirit to the glory of his name."
Piper says more about each of these, but I recommend the book to you and am not interested in just duplicating it here!