A smart player is going to make use of this time. You should take some very deep breaths, easy and even, not forcing any air in or out. At the same time, focus in on a small object. This concentration, combined with your breathing, will bring you heart rate down - so you'll relax and play with your best coordination. Experienced players often focus in on their strings.
Some players actually grunt.
This helps your muscles relax and also increases your power. Saviano points out this technique is borrowed from karate where it is similarly used to release explosive energy. And I've seen these people become dizzy, get nose bleeds, and even pass-out. Just remember to exhale on the exertion, pushing all your breath out from deep down, much like in karate when you go to strike an opponent. The one exception to this rule, according to Haney, are exercises that have a very slow movement and don't call for much energy.