J. R. R. Tolkien wrote a letter to his son talking about how bad preaching can be and then gave some advice on what it takes to have a good sermon. This is great advice for pastors:
The answer to the mystery is prob[ably] not simple; but part of it is that ‘rhetoric’ (of which preaching is a dept.) is an art, which requires (a) some native talent and (b) learning and practice. The instrument used is v[ery] much more complex than a piano, yet most performers are in the position of a man who sits down to a piano and expects to move his audience without any knowledge of the notes at all. The art can be learned (granted some modicum of aptitude) and can then be effective, in a way, when wholly unconnected with sincerity, sanctity, etc. But preaching is complicated by the fact that we expect in it not only a performance, but truth and sincerity, and also at least no word, tone, or note that suggests the possession of vices (such as hypocrisy, vanity) or defects (such as folly, ignorance) in the preacher.
Good sermons require some art, some virtue, some knowledge. Real sermons require some special grace which does not transcend art but arrives at it by instinct or ‘inspiration’; indeed the Holy Spirit seems sometimes to speak through a human mouth providing art, virtue, and insight he does not himself possess: but the occasions are rare. In other times I don’t think an educated person is required to suppress the critical faculty, but it should be kept in order by a constant endeavour to apply the truth (if any), even in cliche form, to oneself exclusively! A difficult exercise… (The Letters of J. R. R. Tolkien, p. 75)