Dr. David Suzuki is watching you. At least, that's what I tell my kids. I play the role of good cop and put David Suzuki in the role of bad cop whenever I witness a waste of energy in the home.
My kids have had the name Suzuki thrown at them their entire lives. They know if they leave a room, they should turn out the lights, because David Suzuki is watching. This threat also applies to recycling, use of the green bin, ensuring the television is off when nobody is watching, keeping the refrigerator door closed and whatever other environmentally irresponsible decisions are made.
Try it for a week and you'll be shocked out how quickly everyone adapts. Nobody wants to disappoint David Suzuki and that dude sees everything.
And the truth is, it is little stuff like that that will essentially make a difference. My wife is awful with it, she will have lights on where she was 2 hours previous and think nothing of it.
I Try to make it a point to go behind her and shut things off because it does make a difference.