I find that getting centered, especially in the midst of turbulence - real or imagined - is an essential first step to performing well. A favourite technique is 'deep water breathing'. It's a great way to take control of the mind by first calming it down and then letting go of the chatter.
Sitting down in a quiet spot (noise canceling headphones work well if you are in a busy place) close your eyes and take six deep full breaths. Feel your breath going in and out, your blood flowing and your muscles relax.
Next, imagine you are sitting on the bottom of a lake, think about how it feels - soft, weightless, still and silent. Looking up what do you see? Crystal clear water, light filtering from above, perhaps some current and bubbles of air rising. Any thoughts that arise are simply ripples, if you have any concerns - let them float up to the surface. If they recur, let them go again and focus on soft deep repetitive breathing.... You might count to ten and back down again.
Practice this every day for a week or two and feel how instantly you can turn down the noise and increase your focus and happiness in those few moments.
Someone said to me, "I don't like it, you can't breath underwater so it doesn't feel 'real'". I thought to myself, half the things I worry about aren't real either, which is precisely why I like this meditation. It clears the gunk out of my brain and lets me at the good stuff!