When you're working at height you will obviously go through all the check-lists to ensure you have the correct equipment. You'll also no-doubt take into account the height and the ground you're working from.
However, have you considered other areas around you?
Sometimes a hazard can come from a place you hadn't planned to be. For example, if you discover you need to make a quick exit from somewhere - is your escape route safe?
Unstable structures, fragile surfaces or other environmental factors could make the job more dangerous, especially when using lifting equipment.
Before working at height, keep in mind not only the area you're currently in, but everywhere around you, too. If you're using lifting equipment, check its route to position and where it will have to go in order to move quickly.
Don't leave anything to chance.