The conventional wisdom today is to read a lot of commentaries, and then to write out a sermon or at least a very full outline putting together what one has learned from the commentaries.
From reading men of the past I learned that they did not approach preaching and teaching that way for the most part. It was common in the past, both pre-reformation and post-reformation, for pastors to preach or teach every single day. Augustine, Chrysostom, Calvin, your average Scottish or puritan minister — they all did it. And yet it simply is not possible to spend 20 hours daily in sermon prep every day. The only conclusion to be gleaned is that they were apparently much more efficient at prepping a sermon. And yet the average Puritan sermon was probably not worse, but better, than what we've come to expect today.
Moreover, that's not just my take on how they saw things. William Fenner, a well-known Puritan preacher, chastised preachers for preaching infrequently (i.e. once or twice a week), and spending too much time preparing each sermon. And period homiletics like those of Zepper and Hoornbeeck discouraged reading many commentaries, and mediocre commentaries, as tending to add more confusion than light. They basically said "read Erasmus on the gospels, Piscator on the Epistles, and you're good to go."
I know not everyone is going to agree with me, but a significant part of the bible is really quite perspicuous, and one reads it and already understands 80% of what it's saying. And a solid commentary will get that to 95%. From there, we have a few options. One is to read thirty more commentaries in an attempt to get that number to 100%. The other option is to move on to what is really the meat of a sermon, which is how to apply the truths in the Scripture to one's own audience and make it stick. (As an aside, the Directory for Public Worship has really good comments on sermon application). My process in preparing a sermon or less is usually the following:
1) I read the passage in Greek.
2) I read it in Greek, again. At this point I don't spend hardly any time in lexicons, exegetical helps, etc, because I know Greek well enough not to need those things. My Hebrew is worse though, so I still need those things for Hebrew.
3) I read one commentary, and if it sheds sufficient light, I leave it at that. If it doesn't seem to really get at the author's meaning all that well, I try another. Matthew Poole's Synopsis is kind of the commentary of last resort here, since he summarizes every other commentator and gives his judgment. In reading the commentary I get a Latin translation as well, which is helpful.
4) I meditate on the text. That's where the Puritans spent most of their homiletical time and energy, and I generally try to do the same. This is really the heart of sermon prep, in my opinion. I think about the following questions: 1) What are the doctrines the author intends to communicate? 2) How is God's glory and honor and the salvation of the elect bound up in what the author is communicating? What does it show us of Christ? 3) If it is a duty the text enjoins, how is God's glory and honor bound up in this duty? 4) What other parts of Scripture shed light on this text and its various points? Usually I have something in mind already, but if not, it's help to check John Brown's notes. 5) What are the consequences of this doctrine for "teaching, correcting, rebuking, training, and consolation" (2 Tim. 3:16; Rom. 15:4)? What doctrines flow from it? What doctrinal errors does it correct directly or by consequence? What sins does it reprove, directly or by consequence? What duties does it encourage or enjoin, directly or by consequence? What consolation does it give the saints? 6) Of these applications, which are the most relevant to me and my audience? 7) What specifically should we do in response? 8) Illustrations: starting from Scripture, is there anything that by way of illustration sheds more light on this, and would be helpful to mention? Anyone who received a similar admonition? Rebuke? Encouragement? Who was moved by the glory of God? If I'm teaching on despair, I would mention Heman (Ps. 88). If I'm talking about the dangers of being unequally yoked, and how it leads one away from Christ, how could I not mention Solomon? etc.
In meditating, I think it's very important to meditate until the Scripture has deeply impressed itself on one's heart, and a fire has been kindled in one's bosom, or the sermon will be cold and dry. Alexander said, and I think this is correct, "Passion is eloquence."
5) I preach or text on the text. I generally do this from memory, without the use of notes. Or if I have any, it's just a bare skeleton. Hodge's sermon outlines are similar in format, if you've seen those. If any illustration or application occurs to me in the moment that hadn't occurred to me in study, I make sure to mention it. These tends to be the best and most natural illustrations. Even though the delivery is ex tempore I make sure to structure the thoughts in a logical way, using the method of Zepper and Hoornbeeck (similar to Jonathan Edwards's layouts, for a comparison), with clear divisions and order.
To be honest, I would never preach this way (extemporaneously, I mean) if it were my own idea to do so, because the very idea of teaching with no notes seems irresponsible. But I read J. W. Alexander's Thoughts on Preaching, and it's what he encouraged, so began the practice, and it's been so much more natural than any time I've tried to teach from a manuscript that I wouldn't consider doing it any other way.
And I think it's not without biblical warrant. Solomon said, "The preparations of the heart belong to man, But the answer of the tongue is from the LORD." And Christ said, "But when they shall lead you, and deliver you up, take no thought beforehand what ye shall speak, neither do ye premeditate: but whatsoever shall be given you in that hour, that speak ye: for it is not ye that speak, but the Holy Ghost."
Now, I suppose one could interpret the latter as being a promise particular to the apostles, but the former is pretty clearly universal in its scope.
Now, if a pastor is full-time, and doesn't have any kids to catechize during the week, or widows to visit, and can give 20 hours to prepping each sermon, I don't think that's wrong by any means, and if the brothers here find it useful that's great. But I do wonder if taking a more puritan approach could help someone out there be both an effective and a frequent preacher. Besides my regular jobs, I teach once at the seminary and twice at church in a week, and I find that the method I've laid out above allows me to edify my listeners, and the comments I get (and I just mean to give an anecdote here, not boast) are almost without exception how well-prepared the lesson was, and how thorough. Now, what they're picking up on isn't how many hours I've spent reading commentaries (which is probably a couple; certainly not dozens), but rather the light shed on the text, the clear manner of presentation, and the relevance of the application to their own lives.
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