The first thing I've done when I moved into my last three apartments was to go through the house and swap out all the lightbulbs in the place. Those spiral bulbs are insanely efficient and really cut down on energy usage.
I unplug adapters of things not in use (since they still drain energy even if they're not plugged into a device), and I let my computers go to sleep after a short time of inactivity. We don't turn the heat up to an unreasonable temperature, which also conserves the amount of fuel we go through, and the amount of energy needed to move all the water around the pipes.
A few little things, but it saves us a lot, and helps the planet more than you'd think.
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