It is important to understand the two essential parts of effective preaching and how each requires its own discipline of preparation.
First, there is the content part of preaching. Preaching is all about accurately exegeting and understanding the truths of the gospel as they unfold in a particular passage of Scripture. I cannot rush this aspect of my preparation. I cannot leave the discipline of content until I have understood the purpose of the passage. And if I cannot practically apply the truths of the passage to my life and to those to whom I will preach, then I haven't yet fully understood the passage. The exegetical process doesn't end with understanding, it ends with application.
It is necessary for me to live with a passage, to carry it around with me, and to marinate my soul with its nourishing and thirst-quenching waters. I simply can't do this in a couple hours. I need meditative time with the passage so the Spirit can work through it in me and through me to the people under my care. I'm about to make some of you angry, but I'm going to say it anyway. If you are developing original content late on a Saturday evening, you have no business preaching it on Sunday. It's unlikely that you will have understood the full range of the radical gospel glories of the passage, it's doubtful that they have confronted your heart, and it's unlikely that you have developed much readiness to communicate them winsomely and practically to your listeners.