According to the video (see above) posted on the Fairfax County Emergency Blog, the important steps to take in an attempt to survive this type of tragedy are run, hide and fight.
The video advises people to run or evacuate the area if possible. It also advises remembering that your safety is more important than your belongings. "Leave your belongings behind," and if possible help others evacuate as well.
Once you are out of the line of fire, try to prevent others from walking into the line of fire and call 911. If you can't get out of the area safely, find a place to hide. Conceal yourself behind large objects out of the shooter's view and do your best to remain quiet and calm.
If you are unable to evacuate or hide, the video suggests fighting and acting with aggression, whether you're working alone or as a group.
The city of Houston produced the emergency video and was funded by a Department of Homeland Security grant. The video depicts a fictional shooting incident in a crowded office building.