A mindfulness meditation can relieve your stress and help you become more focused. It enables you to slow down and refocus your attention away from your stressed out worries. Actually, I've used two versions of this technique, one for emergency use to quickly relieve anxiety, and another for use as a deeper meditation.
Find a quiet, private spot if you can. Take 2 or 3 deep breaths, yawn if you have to. Close your eyes. Now make yourself aware of all the sounds that you hear. Take a mental inventory, listing them in your head as you notice each one.
This is a way of being mindful of the here and now instead of brooding or fretting about your worries. You can do this same technique with your eyes open and focus instead on visual things, like everything that moves, for instance, or use another sense and notice all the scents you smell. Anything that draws your attention to what is right in front of you will work. But the sense of hearing is one of the best since there are often so many sounds happening at once so there is a lot of mindfulness potential. That is, more subtleties to notice, more to be mindful of!
This technique can be done on the spot when you want to calm yourself down quickly, like if you're in panic mode. Or it can be done in a slower, more deliberate way, taking 3 deep breaths, methodically taking inventory of every little sound you hear, one at a time, dwelling on every detail of your surroundings. I've definitely found it to be very calming and to improve my concentration ability to practice mindfulness meditation now and then.
Sometimes a regular activity that you do can be so enjoyable and carefree that you are automatically living fully in the present moment when you engage in it. I realized this was the case for me when I was bike riding with my son. We were leisurely riding to the river one day and I realized how perfectly content and free of worry I was at that moment. I noticed every little breeze, every ray of warm sunshine, the sound of our tires on the sidewalk, the sound of cars... I was totally in the present moment! Think of a moment when you felt this way and make note of what you were doing at the time. Then simply repeat that activity as often as you can, even if just in your mind.