Bhagavad Gita, Chapter 6.10 - 6.17 (traduction by Yogananda)
[ Krishna’s Advice for Successful Practice of Yoga]
6.10 Free from ever-hoping desires and from cravings for possessions, with the heart (waves of feeling) controlled by the soul (by yoga concentration), retiring alone to a quiet place, the yogi should constantly try to unite with the soul.
The aspirant who meditates without eliminating desires and hopes (instigators for actions of sensory enjoyment and possession) finds his mind roaming into the realm of materiality, planning for and visualizing various gains. So when the yogi starts to meditate, he must leave behind all sensory thoughts and all longings for possessions by quieting the waves of feeling (chitta), and the mental restlessness that arises therefrom, through the application of techniques that reinstate the controlling power of the untrammeled superconsciousness of the soul.
6.11 The yogi’s seat, in a clean place, should be firm (not wobbly), neither too hight nor too low, and covered, first, with Kusha grass, then with a deer or tiger skin, then with a cloth.
In the modern world, in both East and West, neither kusha grass nor animal skin is necessary for the meditation seat. (In India it was customary for a forest-dwelling yogi to make his seat on the skin of a tiger or leopard or deer that had died a natural death.) The yogi should meditate on a firm seat, one that is clean―untainted by dirt or unspiritual vibrations of others. The thought or life force emanating from an individual saturates the objects he uses and his dwelling. The proper bodily posture, one which produces calmness in body and mind, is necessary to help the yogi shift his mind from matter to Spirit.
6.12 Established on that seat, concentration the mind on one point, and controlling the activities of the fanciful faculty (chitta, feeling. ―the power that visualizes) and the senses, let him practice yoga for self-purification.
The devotee who sits in a good posture and meditates at the point between the eyebrows learns to practice yoga; in deep concentration, he finds his mind and heart (chitta, feeling) free from sensory distractions and emotional likes and dislikes. He engages in the ultimate "self-purification."
6.13 Firmly holding the spine, neck, and head erect and motionless, let the yogi focus his eyes at the starting place of the nose (the spot between the two eyebrows); let him not gaze around in various direction.
A majority of gita translators and commentators have misinterpreted the word nasikagram to mean "tip of the nose." The word literally means "origin of the nose." The origin or starting place of the nose is the spot between the two eyebrows, the seat of spiritual vision.
6.14 With serenity and fearlessness, with steadfastness in brahmacharya, with the mind controlled, with the thoughts centered on Me, the yogi should sit, meditation on me as the Final Goal.
Straight spine and erectness of the neck and head are important in effective meditation. If one adopts an improper posture―his body bent, or his chin tilted up or down―his crooked vertebrae pinch the spinal nerves. This pressure obstructs the reversed flow of mind and life force from the sensory channels to the brain; there is then no reinforcement of the power of the inner telescopic eye to perceive Omnipresence.
6.15 The self-governed yogi ―he whose mind is fully under control ― thus engaging his soul in ceaseless meditative union with Spirit, attains the peace of My being: the final Nirvana (deliverance).
Lord Krishna speaks of the results one reaps meditating on the Supreme Lord by practising yoga or the science of the individual consciousness attaining communion with the ultimate consciousness. The result is one achieves moksa or liberation.
6.16 O Arjuna! The gourmand, the scanty eater, the person who habitually oversleeps, the one who sleeps too little ―none of these finds success in yoga.
6.17 He who with proper regularity eats, relaxes, works, sleeps, and remains awake will find yoga the destroyer of suffering.
The key word is moderation. Neither too much nor too little of anything.