Although some of the phrasing is a little dusty, and the period needs to be borne in mind, there is some sound advice here about the pursuit of plainness in preaching from William Plumer:
The Word of God must be preached plainly and simply, not in allusions and doubtful terms, not in innuendoes and learned phrases; not in words which man's wisdom teacheth, but in words which the Holy Ghost teacheth; not with the refinements of the schools, but so that the women, and children, and simple people may understand. Baxter said that if ministers had sinned in Latin, he would have written his Reformed Pastor in Latin ; but as they had sinned in plain English, he must write in plain English also. Some of Romaine's people thought his style of preaching too plain and common, and requested him to display a little more learning in the pulpit. Accordingly, the next opportunity, he first read his text in Hebrew, saying, "I suppose scarcely any one in this congregation understands that." He then read it in Greek, and added, "There may be one or two that understand me now. I will next read it in Latin." He did so, and said, "Possibly a few more may comprehend me, but the number is still very limited." Last of all he repeated the text in English, and said, "There, now, you all understand it; which do you think is best? I hope always so to preach that the most ignorant person in the congregation may understand me." Orton says : "I believe many ministers over-polish their sermons. . . . The words of God are those that must reach the heart and do the work." J. Brown, of Haddington, remarks : "So far as I have observed God's dealings with my soul, the flights of preachers have entertained me ; but it was Scripture expressions that did penetrate my soul, and that in a manner peculiar to themselves." Thomas Watson says of the Baptist: "John did not preach so much to please as to profit. He chose rather to show men's sins than his own eloquence. That is the best looking-glass, not which is most gilded, but which shows the truest face." Luther: "To preach simply is high art. Christ does it himself. He speaks of husbandry, of sowing seed, and uses simple peasant's similes. Albrecht Dürer, the famous painter, used to say he 'had no pleasure in pictures that were painted with many colors, but in those that were painted with choice simplicity.' So is it with me as to sermons." The old English bishop was right when he said, "Brethren, it will take all our learning to make things plain." It is not true that "a clear idea is a little idea." Whately says: "Bacon is a striking instance of a genius who could think so profoundly, and at the same time so clearly, that an ordinary man understands readily most of his wisest sayings ; and perhaps thinks them so self-evident as hardly to need mention. But, on reconsideration, you perceive more and more how many important applications one of his maxims will have, and how often it has been overlooked ; and on returning to it again and again, fresh views of its importance will open on you. One of his sayings will be like one of the heavenly bodies that is visible to the naked eye, but in which you see more and more the better the telescope you apply to it. The 'dark sayings' of some other famous writers, on the other hand, may be compared to a fog-bank at sea, which the mariner, at first glance, takes for a chain of majestic mountains, but which, when he turns his glass upon it, proves nothing more than a shapeless heap of unwholesome vapors. When such maxims accordingly are translated into ordinary language, they too often lose the appearance not only of wisdom but of sense. And the attempt to put them into any shape in which they can be intelligently applied to practice is like trying to make a comfortable dress out of some very old piece of brocade, that looks rich and sound in the chest, but when you bring it to the light, and shake out its folds in the air, the colors fly, and the fabric falls to tatters in a moment."
The great object of preaching is the manifestation of the truth so as rightly to impress it on every heart.