You may already know of this but its effectiveness warrants it a place in any list of mental exercises to improve concentration. Sit upright on a chair or on the ground, with your head and back in a straight line. Press your index finger against your right nostril and breathe in gently through your left, mentally ticking off ten seconds. Now remove your finger and exhale through your right nostril, again taking ten seconds to do so. Now repeat the whole procedure with the other finger and left nostril. This exercise should be done a minimum of 20 times at a stretch.

Please refer to the book for more details on this activity.