Although not an ordained minister, I have preached, so along with the other advice given, let me give you two points:
1.) Preach the Word, period. The Word of God will be used as God directs, and God will affect the hearts of hearers accordingly. I used to think that it was dependent upon me and my delivery of the sermon that made a sermon effective. Now in my more mature (and Reformed) years, I realize that that's far from the truth. Yes, we need to practice our oration and delivery, but it's practiced in order to not distract from the Word, and not practiced in order to add to the Word. Don't worry about reactions or results, or about the lack of apparent reactions and results. Just preach the Word. And thank God Almighty that you have this awesome privilege, because that's exactly what it is-a privilege. It's a beautiful calling that exalts the Word of God so plainly and publicly, and should be received by every minister with fear and trembling. Thank God you have that opportunity.
2.) Don't spit when you preach.