Great preachers not only tie together the head and the heart, but they also do two additional things that vault them to greatness. Most preachers have glimpses of Great, but very few can sustain Greatness for longer than a sermon or two. Great preaching is not elusive but Great preachers take the time to develop skills in two areas.
First, Great preachers do not give sermons -- they deliver them. The preacher who is giving a sermon is one who is prepared and well-spoken. The preacher who gives a sermon is one who even thinks about what the congregation needs to hear while prayerfully attuning to the work of the Holy Spirit. The difference in giving a sermon and delivering a sermon is all in how the preacher understands the sermon. Ask anyone who has ever delivered a baby into the world -- the baby is a living thing that has its own movements and actions that are outside the control of anyone else. Anyone who gives a sermon does not understand the sermon is alive and has breath. Delivering a sermon is being keenly aware and flexible enough to move with the sermon as it develops in the moment.
One of the most commonly known Great preachers was Martin Luther King. King knew that he was not giving a sermon but delivering one, which explains why he changed course in the sermon after hearing the voice of Mahalia Jackson shout out, "Tell them about the dream, Martin!" At that point, King's famous sermon began to live in the hearts of all who have heard it. It may be semantics, but there is a very real difference in delivering and giving a sermon, Great preachers know that.
The second thing that Great preachers do (that most of us do not take the time to develop) is something that every stand up comic or author has to have in order to "make it": a well-developed point-of-view (POV). Legendary comic Lenny Bruce had the "how far can we push the First Amendment" POV. The not-as-legendary-but still-okay comic Jeff Foxworthy has the "redneck" point of view. While the greatness of Maria Bamford has the "neurotic eccentric" POV down pat.
Great preachers have a POV: Barbara Brown Taylor has the "sacred in the ordinary" POV. Adam Hamilton has the "speaking to new and nominal Christian group" POV. Fred Craddock has the "story-telling" POV, while MLK Jr. had the POV of "racial and economic justice."
Being able to unite the head and the heart, and deliver sermons from a clear and consistent POV, is what moves good preachers into becoming great preachers.