Generally, here are some best practice guidelines:
Select only one article from a journal issue or one chapter of a book.
Use only two or three articles from the same journal.
Choose new articles or book chapters each semester.
If you want to use several book chapters, put the book on reserve at the library.
Select a textbook for the course and supplement it with selected readings.
Make copies of only required readings, and share a list of optional readings.
Instead of making copies or attaching PDFs to an email, course website or course management page, link to electronic versions of the readings via persistent URLs on library databases or the open Web.
Consider paying permission fees for creating a "course pack" of your readings that you can distribute electronically and use from term to term.