Henrik de Gyor touches upon some of the core resources for metadata planning including the NISO guide to understanding metadata. This comprehensive guide covers metadata standards, creating metadata and other related topics like interoperability.
A newer valuable resource is the Visualization of the Metadata Universe by Jen Riley. “The sheer number of metadata standards in the cultural heritage sector is overwhelming, and their inter-relationships further complicate the situation. This visual map of the metadata landscape is intended to assist planners with the selection and implementation of metadata standards.”
Examining these resources when planning metadata will provide a solid foundation from which you can incorporate and create elements specific to your organization’s needs. One of my favorite tactics is creating a “metadata mashup” by combining elements from several different schemas in a customized manner. For instance, I like to use Dublin Core, IPTC and VRA Core for a lot of image related cataloging.