Switch off lights as you leave the room.
I always do this, having been trained by my parents from a very young age to switch off everything (lights, tv, granny's oxygen supply*) before moving to a different part of the house for any period of time.
So, today we shall have to do something else, and I have just the thing. About the leaving mobile phone chargers on when they're not charging; I always disconnect mine when it's not in use, but when I'm actually charging the phone, quite often it gets left on for ages longer than it takes to charge, because I've no idea how long it takes (usually I leave it overnight). So today I figured out roughly how long my phone takes to charge (about 2 hours), so that in future I can remove it more speedily once it's ready.
* I made that last bit up