If you must go to a gun free zone, have a plan. Know entrances and exits, know what in your environment will reasonably stop bullets, and what in your environment can be used, adapted, or modified into a weapon (think heavy or sharp).
One of the hardest things to make a habit is paying attention to your environment. It's bad enough to be distracted by a friend or one of the many giant TVs positioned around a restaurant or bar, but the age of information allows us to be constantly connected to family, friends, work, and social media. Decide what's more important, keeping track of your environment (safety) or... grabbing for your cell phone every time it pings. Like the big signs say on the interstate in reference to texting and driving, it can wait.
If an active killing event starts, what are your options?
Leave - the farther you are from danger, the farther you are from harm. Knowing how to get out (doors/windows) as well as alternate routes will get you to safety.