How to be calm in a stressfull situation

Identify the cause of your stress. Is your heart pounding because that idiot just cut you off on the freeway, or is it because of that presentation you have to give to your boss this afternoon? Think for a moment and try to figure out what’s really bothering you.
Choose your response. Even if you’re powerless to change the source of your stress, you have the power to choose how you’ll respond to it. The appropriate response to stress should depend on what’s causing it: you can either shake off your stress (ignore it and let it go immediately) or face it head-on. In order to choose your response, ask yourself some questions.
Does it matter

Yeah, it’s all small stuff, but some stuff is smaller than others. Consider how long the source of stress will affect you. That idiot driver will be gone in a moment if you just let him keep speeding down the road, but the death of a loved one may affect you for years.
How much control do you have over the situation?

You can’t control the rain that’s ruining your wedding, but you can control how well you do on your algebra exam tomorrow.
Is the source of stress in the past, present, or future?

You can’t change the past, but you can respond to the present and prepare for the future. Let your past troubles fade.
Shake it all off. If a situation is beyond your control, or if it just isn’t that important, stop worrying about it. Easier said than done? Just do it.
Inhale deeply through your nose.

In your mind, count to five seconds, and then exhale slowly through your mouth, for another five seconds. Repeat this breathing pattern until you feel comfortable with it.
Think about something else.

Get your mind off the stress by thinking about something that makes you happy, such as your kids or spouse (provided they’re not the cause of the current stress), or by concentrating about the things you have planned for the day.
Visualize relaxing things,

such as a deserted island or a country road. Close your eyes and try to picture even minor details about the imaginary place, and you can put yourself in that situation instead of the one you’re in.
Get away from the cause of the stress.

If you can physically escape the stress trigger, do so. Leave the room or pull off the road for a moment to put things in perspective.
Get some exercise.

Whether you go for a run, do calisthenics, do yoga, or lift weights, 10-20 minutes of physical exercise every day can relax you even when "nothing can". Getting plenty of exercise also helps you respond better to stress in the long run.
Face your stress source head-on when you're ready.

Getting stressed is not going to resolve the situation. Sitting around worrying is a good way to procrastinate, but procrastinating will only prolong or intensify the stress. Facing your stress head-on is really just a way to shake off a bad situation that you cannot or should not ignore. If you can change the outcome of a situation that matters to you, the quickest way to overcome that fear or to empower yourself is to take action as quickly as possible. Once you’ve resolved the underlying problem, you can shake off the stress because it no longer matters. The steps below will help you. If you feel paralyzed, use the steps above to relax and temporarily distance yourself from the situation just long enough to be able to see it clearly.
Make a plan

Sometimes you can resolve a stressful situation right away with one action, but often you’ll need several steps, perhaps over a long period. Write out a plan with attainable goals and a time line for reaching those goals. Additionally, many stressful situations are avoidable. If you prepare ahead of time for important events and make contingency plans, you may not have to cope with as much stress later. An ounce of prevention is worth a pound of cure.
Take one step at a time.

A complex problem can be overwhelming, even when you’ve got your plan mapped out, but remember: the journey of a thousand miles begins with one step. Just focus on one small goal at a time.
Be realistic.

If you continue to experience stress because no matter how hard you try you can’t take the steps quickly enough, you probably haven’t set realistic goals. In a culture that values a can-do attitude, it can be hard to accept that sometimes you can’t do something, at least not within a given period of time. If that’s the case, revise your time line or lower your expectations. If you can’t do that, the situation qualifies as one which you can’t control. Learn from your experience, but let it go.