Remember that crises do end. This makes it easier to deal with the current situation. Whatever the outcome, it is unlikely to be that significant. To help yourself put the pressure into perspective, think of the things that really matter.
Decide what you can do and what is outside your control. Focus on areas you can influence; ignore the rest.
Crisis is infectious. Slow your movements, your speech and, if you can, your thoughts. If the people around see you acting calmly they will reflect your manner. This in turn will help you relax.
Change the atmosphere. Go somewhere different, tell a joke, get everyone to stand up (or sit down). Even something as simple as smiling can transform the mood.
Look on the crisis as a challenge rather than as an obstacle. Look at yourself as a hero/heroine with the unique ability to overcome it. Turn the crisis into an adventure rather than a tragedy.
Talk to other people, especially those with experience of similar situations.
Don't bury your emotions - share them with someone, or at least write them down. If you articulate your feelings they are less likely to build up and gush out at the wrong time.
Reflect, make a decision and get on with it. Taking action gives you a sense of progress, distracts you from worrying and helps you feel in control. All this reduces panic and induces calm.
Be comfortable with partial success and limit the expectations of the people around you