Okay, so I'm not as much of an environment freak as I'd like to be, but there are simple things you can do that help the environment, and it makes me so mad when people don't.
The biggest thing is using lights when you don't need to. This annoys me in two ways.
The first way this annoys me is when people turn on lights when there is plenty of light coming in from the windows already. If it's light enough that I can read with the lights off, you should be able to browse the Internet with the lights off.
Secondly, I hate it when people leave a room and leave the lights on. There is no need for the kitchen light to be on if your bedroom light is on! It's completely unnecessary. It's not doing anyone any good.