negative externality

Install double-pane windows in your home
Use CFL or LED light bulbs
Use Energy Star labeled equipment
Turn off all home and office equipment when not in use, and use power strips for your electricity needs; consider a smart power strip
Turn off lights when not in use
Turn down the thermostat - lowering it by just one degree can reduce heating energy costs significantly; consider a smart thermostat
Avoid unnecessary electricity loads (electrical equipment that still uses energy even after being turned off) by using a power strip
Reduce your water heater temperature from 140 degrees to 120 degrees
Consider a Home Energy Management system
Weatherize and upgrade the insulation in your home
Consider adding solar panels to your rooftop, or participating in a community solar program

Reduce, reuse, recycle
Bring reusable shopping bags with you to the grocery store or farmer's market
Support your local farmer's market and buy seasonal farmed foods and produce
Use eco-friendly cleaning supplies
Favor cloth over paper products
Choose natural lawn care instead of using pesticides and synthetic fertilizers
Consider using biodegradable detergent and oxygen bleach
Consider getting in the habit of using reusable mugs, thermoses, sports bottles, etc… daily
Install low-flow toilets and water-saving faucets
Consider bicycling instead of driving if possible...try biking to work or using public transit, where these alternatives are accessible
Instead of a car that relies on conventional gasoline, consider a hybrid or plug-in hybrid, or consider an all-electric vehicle