Do read "click to accept" agreements or "read me" files for clip art, shareware and freeware on the Web to make sure you are aware of any restrictions.
Do buy, license or provide a web link to an image you wish to digitize if it is available for purchase or licensing, or if it's available online. If it's not, you can digitize and use it, but limit access to all images except low-resolution thumbnails to students enrolled in the class. Terminate student access to all such images at the end of the semester.
Do include any copyright notice provided on the original work, and include an appropriate citation and attribution regarding the source of the original work. Include a notice for students that the materials are copyright protected materials and further use is restricted.
Do remember companies like Disney, McDonald's, Mattel and Coca-Cola aggressively monitor the web for infringement. With search engines such as Google, they will find you if you use one of their logos inappropriately and without permission.
Don't use substantial portions or excerpts that can be considered the "heart" of someone else's work.
Don't assume any use is fair use just because you're teaching in a classroom.
Don't repeatedly use other's copyrighted materials without permission. If you plan to use other's copyrighted materials in the same class, semester after semester, you must obtain permission from the owner of the copyrighted materials.
Don't substitute copying for the purchase of books, publishers' reprints, workbooks or periodicals.
Don't use copying to create, replace or substitute for anthologies, compilations or collective works. This applies to whether you use accumulated works or if you reproduce and use the works separately.
Don't copy works intended to be "consumable" in the course of study, such as workbooks, exercises, standardized tests, answer sheets, etc.