Be aware of your environment and any possible dangers
Take note of the two nearest exits in any facility you visit
If you are in an office, stay there and secure the door
If you are in a hallway, get into a room and secure the door
When the shooter is at close range and you cannot flee, your chance of survival is much greater if you try to incapacitate him/her
As a last resort, attempt to take the active shooter down. When the shooter is at close range and you cannot flee, your chance of survival is much greater if you try to incapacitate him/her.