It seems with even the best of intentions, copyright law is so complex that it is very easy to violate, even with the most moral of intentions. The best best may be to always link to a location where the copyright holder has posted the work, or to ask permission through copyrightl.com. Particularly, I don't think most instructors know how limited "Fair Use" is or that they can only use something one time in one class, not in repeated courses, and that they may only use a small percentage of the work.
