I first try to listen or feel my heart beat. Then I breath in through my nose for 4 beats, hold my breath for 2, breath out for 4 and keep may lungs empty for 2. I breath like this for a few minutes until I feel calm and relax.
Do this exercise regularly before playing or at any time during the day. It will calm you and help you focus on the moment. It will put you in a good state of mind and increase your concentration to maximize the efficiency of your practicing time.