If that is too elaborate a procedure, try this:
Relax and close the eyes.
Smile ever so slightly at the corners of your mouth.
Concentrate on the inner feeling of smiling.
Feel like a smile.
Then send the smile down into your feet.
Smile in your feet.
Feel like your feet are smiling.
You can call up the inner smile anytime in the day, especially when feeling stressed. The trick is to remember to do so, and it takes some practice at watching over yourself.
When you have spare time, you can smile into your feet to engage them directly in the de-stressing.
No theories or books are necessary for this practice.