Highly recommend conscious breath meditation to practise calm mind, it's as simple as sitting in a chair eyes closed and just focusing on the life breath leaving and entering your body at the nostrils, you will find your mind falls silent with practise.
With more practise you can do this at any time in life just by focusing on the breath, very handy if you are trying to motivate yourself to do something like exercise, having a calm almost blank mind really helps silence any voice talking against working out and you just do it, it also silences cravings for junk food :)
Meditation overall is the best thing you can do for your life really, not just workout, buddhist or yogic meditation techniques are good, but don't worry about technique, the simplest one's are often best.
Wimhoff breathing is great for getting more power in your workout.