Probably the most ancient trick in the world: breathing! Take 2 or 3 deep breaths, feel some stress flowing out and sense your inner wisdom being allowed to guide you for a second.
So many authors bringing this up are not too many. We can always be reminded once more of this simple yet effective trick: to breath and let some inner guidance lend you a hand. This one you really have to test physically.
Note: if it works magically, you might just be tempted to say: "I have to do this all the time"! Well, I don't believe much in disciplined practices - I'm not much for any "thought police" methods - so that's not really what I'm pointing towards. Besides, it will probably loose its power if overused.
Just know that a shift in mental state can go a long way in perspective and performance. Anytime soon another author will ring the "breath bell" once more and that will be a great time to use it again. It should suffice...
"Great thoughts come to a quiet mind"