April 19, 2013 -- intro: In 2010 there were 30 "active shooter" attacks in the U.S., according to John Bruner, a 20-year law-enforcement veteran who is the former Chief County Detective with the Greene County (Pa.) District Attorney's Office. That's more than the 1960s and 1970s combined, he added.

Since the Aurora movie theater shootings, in July 2012, there have been 35 active shooters, he said, "which is unprecedented."

"These numbers, sadly, are going up, and so we have to get out of that surreal moment [of shock or disbelief] and realize that ... this could happen," Bruner said.

Watch the full story on "20/20: In an Instant" Friday at 10 ET

quicklist: title: Prepare text: In emergencies, Bruner said, "neurologically we go through a fight or flight process. Our frontal cortex, which is our deliberate thought that we experience every day, that shuts down."

That often causes tunnel vision and auditory exclusion -- impaired hearing due to the fight-or-flight state.

"But what kicks in is your ventral striatum," Bruner went on. "That's where your muscle memory is stored. So if you train for these situations, and you train properly, you will strengthen that muscle memory."

Bruner advocates widespread frequent drills akin to the frequent fire drills in the 1970s and bomb shelter drills in the 1950s.

"We have to practice this every eight months or so, especially with businesses with high turnover," Bruner said. "You've got to get a plan together."

quicklist: title: Run -- and Take Others With You text: Some people's first instinct may be to freeze and hide, but if possible, you should fight it and get yourself and others out, Bruner said.

"If you know the shooter is pretty far away," Bruner said, "encourage people, Hey, join us, we have a shooter down the hall. Let's exit over here."

If there's an elderly person who may be disoriented, grab him or her if he or she is within arm's length, Bruner added.

quicklist: title: Leave the Cellphone text: "You want to go," Bruner said. "The last thing you want to worry about is your cellphone. You want to leave that building."

quicklist: title: Can't Run? Hide text: "If you can't exit, and you know your threat is in between you and the exit, then you're going to have to hide," Bruner said.

Then do all you can to prevent the shooter from entering the room and causing injury, Bruner said. Lock the door and use a doorstop if there is one. Put a chair, a sofa, and even a desk in front of the door. Turn the lights off.

The more weight and objects, the greater the distraction to the shooter trying to get in. This not only makes it harder for him to shoot at you, Bruner said, it lets time be your ally.

"Two or three minutes in an active shooter [situation] is a lot of time," Bruner said. "[It] allows our police to come in and do their job."

quicklist: title: Silence Your Cellphone text: "When you get everything set up, you're going to ... silence your cellphone," Bruner said. Use a landline and call 9-1-1 and, whispering, let them know what floor you're on and what you're seeing and hearing, Bruner said.

quicklist: title: Why the Landline? text: "If you're calling from a landline, and something happens and you drop the phone, [the address] is going to pop up in your dispatch center," Bruner said. "If you call on your cellphone, that information isn't going to pop up on their computer screens. So that may buy an extra 15 seconds if they can say, Hey, we have an unknown problem at whatever this address is."

quicklist: title: Fight text: If it comes to that, "you have to fight for your life," Bruner said. "You have to develop a survival warrior mindset ... and you're going to utilize everything you can as a weapon."

Scissors, hot coffee, the coffee carafe, other glass objects from an office pantry, for example. Fire extinguishers, either engaged or as a weapon to inflict blunt force trauma, are good.

"Maybe you keep a pen on you. You back it with your thumb so you can really make it an effective tool to help save your life," Bruner said.

quicklist: title: Forget About Getting Shot -- Fight text: "Most people who are shot by a gun survive. So understand that and cling to that. Just keep fighting. ... You want to buy time. You want to distract this person, because it's going to take police three or four minutes to get [there]."

quicklist: title: Aim High text: "You go for any piece of body you can," Bruner said. "The most important and most dangerous part of that man is his hands and the gun."

If you can't do that, aim for the eyes, face, shoulder and neck, Bruner said. quicklist: title: Fight as a Group text: "You want to distract him. If there's other people [around] that can assist you, that's where this mob mentality has to come in."

quicklist: title: Whatever You Do, Do Something text: "The first five seconds of an active shooter incident [are] paramount," Bruner said. Don't freeze in disbelief. "React immediately."

"The game has changed," Bruner said. "This country is experiencing a lot of traumatic incidents, as we well know, so you have to be prepared."

Watch the full story on "20/20: In an Instant" Friday at 10 ET