http://info.med.yale.edu/caim/StyleManual_Top.HTML
http://www.dsiegel.com/tips/index.html
http://www.highfive.com/
Also recommended is Edward Tufte's book "Envisioning Information". His other
book "The Visual Display of Quantitative Information" is also good, but it
is more focused toward engineering or data-intensive presentations.
Nate