This article, "Thirteen Tips for Effective Tagging" was recently updated and published on TechSoup. It morphs two different blog posts by nonprofit tagging thought leaders, Ruby Sinreich and Alexandra Samuel.
These are excellent practical tips for choosing tags - either if you're doing this for your own personal information management or to use as a guide to help your organization come up with a tagging policy or at least some standard tags.
UPDATE: I've published a resource on tagging and nonprofits. If you're just discovering my blog or tagging for nonprofits via this post, check out my Nonprofit Tagging Wikispace.