How to Survive an Active Shooter

First and foremost develop a "Survivor's Mindset" which is one of the most valuable pieces of advise we've heard.  Be prepared and know what's going on around you and how to survive it.  The Houston Police Dept (HPD) in conjunction with Homeland Security have developed a YouTube video: Run, Hide, Fight.  It only takes a few minutes to watch this.
Officer Stephen Daniel of the HPD summariezes it this way:

“Active shooter” incidents — like the one last summer in Aurora, Colo., and more recently in Milwaukee — have forced law enforcement to re-evaluate training and tactics.

And they tell those who find themselves in an active shooter situation to run, hide, fight.

Run — Escape if possible. Encourage and help others, but get yourself out regardless of whether others agree to follow.

Hide — Conceal yourself out of the shooter’s view, behind large objects when possible.

Fight — As a last resort and only if your life depends on it. But commit to your actions.