Michael Kruger has a good post here emphasizing the importance of asking and answering the question, “What did you leave out?” I think he is exactly right and exactly write. I still find it painful to excise content that I have garnered through my studious blood, sweat, and tears because it doesn't fit a particular message for a particular audience.
One way that I ease my pain is to create two versions of my sermon. One document contains everything including the kitchen sink and another file has the message that I will actually preach. By doing this, I don't feel that I am throwing anything away although in actual practice I have rarely revisited and used this information.
On a related note, I often do a post-preaching analysis where I often note more of what could have been left out rather than what I should have included. These notes are helpful if/when I ever preach the message again.