Write a theme sentence first

Theme? Theme? I don' need no stinkin' theme!

Updated and revised: 6/6/12 11 AM:

A theme sentence is a powerful and necessary tool for writing nonfiction.

It is used to guide both writing and subsequent revision (because everything is revised). If something has to be cut, the theme shows what can go. If something needs to be added, the theme sentence shows what is missing.

There are three big problems in coming up with a good theme sentence:

1. Writing the theme sentence too early.
The underlying theme of any piece of writing may only become clear to the writer after time-consuming and painful research. When writing something long--say over 5,000 words--the writer may not know the theme until he or she has done considerable thinking, analyzing, reading, and note taking. Writing the theme sentence too early tends to limit the research, and stifle the writing.

2. Writing the theme sentence too late.
Writing the theme sentence too late often leads to bloated drafts that require major cuts during revision. A bloated draft is not always bad. One way to find the underlying theme is to write a sprawling first draft. This is a perfectly acceptable way to find a theme, it's just a lot of work. It may mean writing one, two, or a dozen drafts of a book before the theme emerges. 

3. Incomplete theme sentences.
The most effective theme sentences are fairly short. They have a Subject Verb Object construction,

A complete theme sentence might be: "Theme Directs Writing" The subject (theme) does something (directs) to the object (writing.) What is important? Finding the theme. Why is it important? Because the theme directs the writing. What's the point of this blog post? To show how a theme sentence directs writing, and why it is important to find it before beginning the first draft.

An incomplete theme sentence might be, "Theme is really important to writing."  How is it important? Why is it important? What place does a theme sentence play in the writing?  What's the point of this blog post? None of these questions are answered by an incomplete theme sentence.

For a superb description of the theme sentence see Jack Hart's  Storycraft.