Try taking several long, deep, mindful breathes. Breathing is at the core of yoga. I discovered yoga 14 years ago and don't know what I'd do without it. While a regular one-hour yoga class may not be feasible for everyone at first, one can easily incorporate breathing as part of a relaxation process because we all do it every day, every minute. It's already a part of us.
When you are feeling stressed just immediately start concentrating on your breathing. Notice it, How does it feel? Short and choppy? Are you breathing through your mouth? Breathing though the mouth is one of the first physical signs of stress. The mind can only concentrate on one thing at a time so by focusing on your breath, you can slow it down, and that way you force your mind to slow down, too. Breathe in slowly and deeply through your nose only, and exhale just as slowly and deeply making sure all the air is pushed out. Then begin again. Try it! It does work, and the great thing about it is that it costs nothing and you can do it anywhere anytime.