how do you get someone to turn off the lights when they leave the room?

How old is your little brother? Is he old enough to help cover some of the bills? If he doesn't turn off the lights, charge him! Let him know about the importance of "going green." If he doesn't turn off the light, wait until he's far away from it and then make him go all the way back and turn it off.

Light bills can be very expensive. If you are tired of telling him, give him consequences for not obeying you.