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.