Proverbs 22:3 Know when to stop

The prudent see danger and take refuge, but the simple keep going and pay the penalty.

A very practical proverb like this one has more than one application for the preacher. It is undoubtedly good to preach series of sermons from books of the Bible but if you find your series wearying the congregation, better to leave it and move on to something else. One reason Spurgeon gave for avoiding lengthy series was his boyhood experience with a preacher's interminable series on Hebrews.

He recalled

I have a very lively, or rather a deadly, recollection of a certain series of discourses on the Hebrews, which made a deep impression on my mind of the most undesirable kind. I wished frequently that the Hebrews had kept the Epistle to themselves, for it sadly bored one poor Gentile lad. By the time the seventh or eighth discourse had been delivered, only the very good people could stand it: these, of course, declared that they never heard more valuable expositions, but to those of a more carnal judgement it appeared that each sermon increased in dullness. Paul, in that Epistle, exhorts us to suffer the word of exhortation, and we did so. I also recollect hearing in my younger days long passages out of Daniel, which might have been exceedingly instructive to me if I had obtained the remotest conception of what they meant.

I heard of a man who never announced a series but would always say he was preaching from Chapter 1 and see how it went.

The same danger lurks in the individual sermon. Martin Luther warns “Some plague the people with too long sermons; for the faculty of listening is a tender thing, and soon becomes weary and satiated.” Prayers are another area where the proverb could apply. Spurgeon again complained about people who "pray us into a good frame of mind and heart, and then, by their long prayer, pray us out of it again". He recalls John MacDonald saying “When I am in a bad frame of mind I always pray short prayers, because my prayer will not be of any use, and when I am in a good frame of mind and heart, I pray short prayers, because if other people are in a good frame too, I might, if I kept on longer, pray them into a bad frame.”
An anecdote is told of D L Moody that in one of his meetings in the East End in 1885 someone was leading in prayer but went on too long. After a while, Moody stood and said, "Let us sing a hymn while our brother finishes his prayer." The source of the story is Dr. W. T. Grenfell. In his autobiography, A Labrador Doctor he tells how the prayer had so wearied him he was about to leave. Moody's prompt action kept Grenfell there and he was converted and went on to be a great medical missionary. 

Proverbs 23:9 Ignore fools

Do not speak to fools, for they will scorn your prudent words

When we preach we want to cast the net as widely as is possible. However, we need to recognise that there are some whom it is impossible to reach through the ordinary preaching of the Word. Fools, that is those who are morally deficient and without faith and here wilfully and persistently deaf to the gospel, cannot be reached by this means. Such people will scorn your prudent words. They will not listen. This is why Jesus (Matthew 7:6) warned his disciples not to throw pigs to pearls or what is sacred to dogs. Indeed, he suggests that it may be worse than the proverb intimates - they may turn on you and harm you. 1 Corinthians 1:18 says that the message of the cross is foolishness to those who are perishing and so although God himself can break through preaching to fools cannot of itself. From time to time people will turn up at church who have no intention of doing anything but scorning the Word. They will not be persuaded.