In her book on disaster survival, Amanda Ripley (2008) identifies the common response patterns of people in disaster situations and reveals that three phases of response are commonly seen - denial, deliberation, and the decisive moment. 

Denial

Contrary to the common perception of people panicking and stampeding during a disaster, Ripley found that it was more common for people to deny that a disaster was even happening. Often people spent time speaking to others about what was happening and gathering belongings, rather than evacuating. 

Ripley attributes this reaction to normalcy bias. That is, our brains tend to interpret information as if it is part of our everyday experience.  Because of this, people tend to underestimate both the likelihood of an emergency and the possible effects of an emergency. It takes time for the brain to process this novel information and recognize it as a life-threatening situation.

Deliberation

At this point, people in an emergency have to decide what to do. If the person does not have a preexisting plan, this creates a serious problem because the effects of life-threatening stress on your bodily systems severely limit your ability to both perceive information and make plans, we know this condition as panic.  It will take a conscious effort to overcome panic and act.  To overcome panic you should;

Breathe: Breathing through the nose for a three count, holding the breath for a two count, breathing out for a three count, and then pausing for a two count before beginning the next breath has been shown to lower people's heart rates dramatically for a short period of time and can help circumvent panic.

Shift the Emotion: When experiencing feelings of panic and fear, it is easier to shift the fear response to anger than it is to restore control. Don't get scared. Get mad!

OODA Loop: Observe, Orient, Decide, Act.  Use the OODA Loop to focus your thinking and move you toward action. 

Willpower: In the case of a violent encounter, your brain is setting off a variety of panic alarms. By exerting willpower, a person is trying to make their brain override these alarms. This can be done, but it takes conscious effort. Willpower is, however, a limited resource, it can prevent or delay some stressful situations but will eventually fail.  

Move: Freezing is almost always the wrong response. It leads to a feeling of helplessness. When people feel helpless, their stress levels increase, which further hinders functioning.  Taking action - any action - can help give a sense of control and help reduce stress responses. Just getting to your feet or taking a fighting stance will help you focus.

The Decisive Moment

Once a decision has been made, act quickly and decisively. Failure to act quickly can result in your remaining in a position to be injured or killed during an emergency. The faster we can get through the phases of Denial and Deliberation, the quicker we will reach the Decisive Moment and begin to take actions that can save our lives and the lives of those around us.

Prepare Critical Skills to Function:  Train a skill to the point where it is automatic.  Once you master a skill, it becomes an automatic function.

Develop Action Scripts: Finally, you can train your brain when you are NOT under stress by thinking about what you should do in a stressful situation. It is possible to think through likely scenarios and the appropriate responses to those scenarios to prepare action scripts. When under stress, you can then access these prepared plans. The plan you have thought through beforehand is likely to be of better quality than one you come up with on the spot, effectively programming yourself when you are not under stress to do a better job when you are.