The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit, educational purposes.
The nature of the copyrighted work.
The amount and substantiality of the portion used in relation to the copyright protected work as a whole.
The smaller the percentage of the work used, fair use may be more indicated.
However, if even a small percentage that represents "the heart of the work" is used, fair use may be less likely to be indicated.
The effect of the use on the potential market for or value of the copyright protected work.
WHEN IN DOUBT, ASK FOR PERMISSION.