Enviably Green

Increase attic insulation to a minimum of 18 inches, whatever the insulating material is.
If possible, give away unwanted appliances, electronics, furniture, etc., so that these items can be reused (check out FreeCycle.org).
When buying or building a house, or upgrading appliances, give energy efficiency high priority in the decision process. 
Unplug unused appliances and devices, which consume energy even when turned off.
Consider buying a smaller or more fuel-efficient car when the time comes. 
Replace as many lights as possible with compact fluorescent or LED lights. (Note: dimmable compact fluorescents ARE available.)
Recycle as much as possible. ("Precycle," too; consider which products to buy based on their recycling potential.)
Conserve water, especially on lawns and gardens (think xeriscaping). 
Stay politically attuned. Lawmakers and other officials often fail to pass or enforce meaningful environmental legislation because big lobbies pressure them, but they don't often hear from individuals. 
Pray and ask God to open our eyes on the issue of sustainability. Sacrifices are easier to make and bear when we know we are glorifying God through our actions, and easier still in the company of a like-minded community.
 

I hasten to note that my top 10 doesn't include "purchase a renewable energy system." While that would be commendable, the benefit of a household-scale photovoltaic, wind or solar heating system would be wasted without first taking the preceding steps.