1) Only wash full loads of dishes
2) Dont pre-rinse your dishes before putting them in the dishwasher
If you really want to be energy efficient about it dont use the dry cycle, but instead leave your dishwasher open overnight to let them air dry.
According to the University of Bonn Germany research project (pdf), a dishwasher uses half the energy, one sixth the water and much less soap than hand washing. Newer Energy efficient dishwashers with the Energy Star Label can save a homeowner up to $25 per year over older models. Energy efficient units clean better use up to 30% less water and use less soap.
Read more on this topic by reading Larry Wests article at About.com. Larry West is an environmental expert, that is an environmental topic guide on About.com