I've had Humani Generis Redemptionem sitting on my desktop for a while now and it's time I did something with it. Although it was promulgated by Benedict XV in 1917, it is quite topical and could easily apply to the current generation of televangelists. Speaking of the failure of modern preaching he says
If that weapon does not everywhere produce its effect, the blame certainly must be laid on those ministers of the Gospel who do not handle it as they should. For no one can maintain that the Apostles were living in better times than ours, that they found minds more readily disposed towards the Gospel or that they met with less opposition to the law of God.
The first thing he does is set out the office of bishop as the successor to the apostles and the source of preaching and teaching. The bishop sends forth preachers, they are not self appointed.
Let this then be the first law laid down: that no one on his own responsibility undertake the office of preaching. In order to fulfill that duty everyone must have a lawful mission, and that mission can be conferred by the Bishop alone.
The lecture circuit in the early 20th century served much the same purpose as modern mass media does today. A search of such names as Chautauqua or Lyceum will yield some background on this. Apparently itinerant preachers were a particular problem as was the vulnerability of some churches to unauthorized preaching.
For the man who owing to his peculiar bent of mind, or any other cause, should choose to undertake the ministry of the Word, finds easy access to the pulpits of our churches as to a drill-ground where any one may practice at will. Therefore, Venerable Brethren, it is your duty to see that such a grave abuse should disappear, and since you will have to render to God and to His Church an account of the manner in which you feed your flock, allow no one to creep unbidden into the sheepfold and to feed the sheep of Christ according to his fancy. Therefore let no one henceforth preach in your dioceses except on your summons and with your approval.
He makes the point that some gospel truths are difficult to hear and that there are preachers who avoid these unpopular topics in favor of the cause celebre of the day.
What efforts do such men make to acquire reputation by their sermons from the size and wealth of the cities and splendor of the great churches in which they preach? But since among the truths revealed by God there are some which frighten the weakness of our corrupt nature, and which therefore are not calculated to attract the multitude, they carefully avoid them, and treat themes, in which, the place accepted, there is nothing sacred. Not seldom it happens that in the very midst of a discourse upon the things of eternity, they turn to politics, particularly if any questions of this kind just then deeply engross the minds of their hearers. They seem to have only one aim, to please their hearers and curry favor with those whom St. Paul describes as "having itching ears."
Those having itchy ears can be found around us today. How many are devoted to this preacher or that celebrity because they hear what they want to hear. These devotees become prey to more unscrupulous preachers whose main interest seems to be soliciting donations.
Forgetting the saying of Gregory: "The priest does not preach that he may eat, but should eat that he may preach," [In I Regum, lib. iii], there are not a few who, because they think that they are unsuited for other labors by which they might be decently supported, take to preaching, not that they may worthily exercise the sacred ministry, but to make money.
I strongly recommend reading the entire encyclical which can be found along with all the others at the link on the right. No, I'm not going to link to it here... go look. The website is worth a browse.