There are many great resources to help you understand the concepts of Fair Use and how to apply them.
Many organizations have created their own best practices guidelines to address their own disciplines. Here you will find links to several discipline-specific guides to adhering to Fair Use guidelines:
In determining fair use, you should weigh the benefit to society against the author's reward. Here are the four factors that can determine fair use and some questions to think about:
1. The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes
Is it a transformative? Are you using the material in a new way?
Is the use for teaching or scholarship?
What is the intent behind the usage?
2. The nature of the copyrighted work
Is it a work of fiction or non-fiction?
Is it educational or commercial?
Is the work factual or creative?
3. The amount and substantiality of the portion used in relation to the copyrighted work as a whole
How much of the work do you really need?
Are you using the heart of the work?
4. The effect of the use upon the potential market for, or value of, the copyrighted work
Would the publisher/author be harmed financially because this would replace sales of their work?
How would this use affect the author if everyone did this?