As a community, we should strive to obey not just the legal standards of copyright but the ethical standards as well. The easiest ethical test to apply is the Golden Rule. It is therefore appropriate to ask how you would feel about someone using works to which you held copyright.
Suppose a colleague at another institution asked your permission to make copies of one of your journal articles to hand out to a class. How would you feel? If you are like most people, you would probably feel honored. We do not write journal articles for monetary gain; we mean for them to be shared with the scholarly community.
Now, suppose you published a book and a colleague at another institution asked permission to scan the entire book and make it available to students through a course management system. How would you feel? If you are like most people, you would probably prefer that the students buy the book!
Short stories, poems, photographs - you can apply this simple ethical test to countless other examples. You will probably find it easy to decide when it would be fine for somebody else to copy your work, when they should have to pay a fee for each copy, and when it would be appropriate for their institution to purchase one copy and let multiple students use it. You can then apply the same ethical standards to your own copying and scanning of materials for your classes.
The Golden Rule has no legal standing, but it is an appropriate guide to making decisions that are ethical as well as legal.