Attribute the original authors. [July 23, 2012, 8:56 a.m.]
Building on the work of others is awesome. And recognizing those you're building on is a great practice for encouraging sharing of more resources. "Citation" and "Attribution" are often used as synonyms, but they mean two different things. Citation is a scholarly practice for tracking the ideological underpinnings of a work, usually referencing sources like published books, articles, government documents, primary sources, etc. Citation is a norm, not a legal requirement. Attribution is about crediting a copyright holder according to the terms of a copyright license, usually crediting artistic works like music, fiction, video, and photography. Attribution is a legal condition of using a licensed work.
See http://wiki.creativecommons.org/FAQ#How_do_I_properly_attribute_a_work_offered_under_a_Creative_Commons_license.3F and the CC wiki http://wiki.creativecommons.org/Marking/Users for best practices on providing attribution. Note that if you edited or changed a resource, you need to make note of that in your attribution.
What challenges did you run into while providing attributions? Do you think the CC wiki best practices for attribution can be improved? Suggest changes at the wiki's Talk page: http://wiki.creativecommons.org/index.php?title=Talk:Marking/Users&action=edit&redlink=1
If you are interested in licensing your resulting work, see the challenge on License compatibility (TBD).