Peter Mead, at Biblical Preaching.Net, gives two options for turning a text into a half hour sermon in this article. Mead provides two options. The first is to carefully plan how to drive the main point into your people. This would include tactically deciding how you will present the idea. Mead succinctly summarizes this option as follows:
Option 1 is to take half-an-hour and make that main idea so clear, so transformative, so evident from the text, so applicational for each life.
Option 2 is to simply throw together information about the text, or other things irrelevant to the point, that obfuscates the point of the message.
I think this is an important thing to ask about everything that we are preaching. Is the tidbit that I am about to add to the sermon actually helping to drive the main point home, or apply the point, or celebrate the point? If it is not, then set it aside. Over the course of your preaching ministry you will probably have a chance to brig it back, but it is not a helpful part of the sermon you are currently working on unless it serves the point of your sermon.