If you haven't read Tufte's book on visualization, do it. There are lots of good ideas there; and some that may not be too great. It's not so much about the how as about the concept of a good visualization.
Another excellent source is the New York Times's style manual for visualizations. There are also books written about it. I once heard the graphics style editor of the NYT speak, and she had many good suggestions that they learned from hard experience.
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Bruce Hartman
Professor
University of St. Francis
Tucson, AZ United States
bruce@ahartman.netwebsite:http://drbrucehartman.net/brucewebsite/
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