O'Reilly Network: Open Source Beyond Software [Aug. 01, 2000] How to copyleft your work

You can copyleft any kind of work that is recognized by copyright law, provided the work exists in machine-readable form (where identical copies can be made without harm to the original), and provided that you are its copyright holder. If you are the author of a work, then you automatically hold its copyright; you don't need to register your work with the copyright office.
The Design Science License is a generalized "copyleft" license that was designed to be used by the general public for such works.