Turn off the light!
Just like I am sure your mom always told you to not stand in front of an open fridge, I am sure she always told you to turn the light off when you leave a room. And your mama was right!
If there is no one in a room, there is no point in having a light on in that room. I will admit that I am guilty of leaving lights on when they are not needed, but I still try to make a point of flipping off the switch as much as possible.
But leaving the light off doesn't have to only apply to unoccupied rooms. What about leaving the light off when you are IN the room? Sunshine provides great lighting during the day and is free. I have heard of people who even make a sort of game out of seeing how long they can go during the day without turning on a single light. I try to do that as much as possible some days. Which is fine by the husband - he hates light!