Even if you only have five minutes, go for a walk. You just need to take yourself away from an instant source of stress to calm down. All you need is a few minutes to organize your thoughts.
If I'm tired, seriously upset or stressed, one of the best ways to relax is a book. Fiction fits perfectly (especially something funny and uplifting). Reading fiction takes you from the present to another world. You can forget about all the problems for a few minutes and focus on the story.
Sometimes a good way to get rid of stress quickly is to just stop. Many teachers and stress counsellors recommend meditation: there is no mystery about it, just sit down and try to clear your mind of thoughts.
I don't know about you, but I can't feel stressed when I laugh. If you have a favorite clip on YouTube, just give yourself a couple of minutes and laugh.
If you feel too much stress, caffeine won't help. But a warm mug with soothing herbal tea will be just right. The very process of its welding can be useful.
Depending on where you are and also on your character, a few blows to the pillow may help. Imagine how stress leaves you through every punch.
During stress, we breathe more often, in small shallow breaths. Concentrate on your breathing (you can do this together with meditation or prayer classes). Breathe with your stomach, not your chest.