I remember my Homiletics courses where we were encouraged to have a clear Dominant Thought. I've gotten away in my teaching and thinking about the importance of a succinct summary. My professor, when preaching, would often say, "if I can wrap up my sermon in a sentence I would say ____________".
I have a conviction that no sermon is ready for preaching, not ready for writing out, until we can express its theme in a short, pregnant sentence as clear as a crystal. I find the getting of that sentence is the hardest, the most exacting, and the most fruitful labour in my study. To compel oneself to fashion that sentence, to dismiss every word that is vague, ragged, ambiguous, to think
oneself through to a form of words which defines the theme with scrupulous exactness—this is surely one of the most vital and essential factors in the making of a sermon: and I do not think any sermon ought to be preached or even written, until that sentence has emerged, clear and lucid as a cloudless moon.”
—J. H. Jowett, The Preacher: His Life and Work (Harper & Bros,
1912), p. 133.
Also I once heard another youth pastor at a conference say that if you can't communicate what you want to say in 18 minutes then you still need to work on your message.
Since I'm no Mark Driscoll, I bore myself after 20 minutes, I need to work harder to achieve this goal.