Michael D. Murray has self-archived "What is Transformative? An Explanatory Synthesis of the Convergence of Transformation and Predominant Purpose in Copyright Fair Use Law."
Here's an excerpt:
The lessons of the transformative test for those engaged in creative, artistic, or literary pursuits may be summed up in the following: if you copy an original work, use it for a different purpose than the purpose for which the original work was created. Modify the contents, function, and meaning of the original work through alteration of the original expression or the addition of significant new expression.