If the previous relaxation technique isn't practical, perhaps because you're at work in an open plan office, create some physical and emotional distance from the stressor. Walk away and find a quiet place even if that's sitting in your car or a bathroom.
Take a couple of deeper breaths and imagine yourself far away. See the stressor for what it is. See it as small and manageable and notice how much calmer you feel. You are much more able to think clearly, to consider the problem or stressor calmly and rationally, and plan what you are going to do about it.
Stress is a medical condition and it can have a serious effect on your health. If you think you are suffering from stress it is crucial that you consult a medical professional.