In Chapter 6 of the Bhagavad Gita, Krishna describes the Practice of Meditation:
"Those who aspire to the state of yoga should seek the Self in inner solitude through meditation. With body and mind controlled they should constantly practice one-pointedness, free from expectations and attachment to material possessions.
Select a clean spot, neither too high nor too low, and seat yourself firmly on a cloth, a deerskin, and kusha grass. Then, once seated, strive to still your thoughts. Make you mind one-pointed in meditation, and your heart will be purified. Hold your body, head, and neck firmly in a straight line, and keep your eyes from wandering. With all fears dissolved in the peace of the Self and all actions dedicated to Brahman, controlling the mind and fixing it on me, sit in meditation with me as your only goal. With senses and mind constantly controlled through meditation, united with the Self within, an aspirant attains nirvana, the state of abiding joy and peace in me.
Arjuna, those who eat too much or eat too little, who sleep too much or sleep too little, will not succeed in meditation. But those who are temperate in eating and sleeping, work and recreation, will come to the end of sorrow through meditation. Through constant effort they learn to withdraw the mind from selfish cravings and absorb it in the Self. Thus they attain the state of union.
When meditation is mastered, the mind is unwavering like the flame of a lamp in a windless place. In the still mind, in the depths of meditation, the Self reveals itself. Beholding the Self by means of the Self, an aspirant knows the joy and peace of complete fulfillment. Having attained that abiding joy beyond the senses, revealed in the still mind, they never swerve from the eternal truth. They desire nothing else and cannot be shaken by the heaviest burden of sorrow.
The practice of meditation frees one from all affliction. This is the path of yoga. Follow it with determination and sustained enthusiasm. Renouncing wholeheartedly all selfish desires and expectations, use your will to control the senses. Little by little, through patience and repeated effort, the mind will become still in the Self.
Whenever the mind wanders, restless and diffuse in its search for satisfaction without, lead it within; train it to rest in the Self. Abiding joy comes to those who still the mind. Freeing themselves from the taint of self-will, with their consciousness unified, they become one with Brahman."
– The Bhagavad Gita, 6:10-27
If there is one passage that sums up the Gita, this is it. By experiencing the Atman, the Self, one becomes completely fulfilled, and is thus capable of acting in the world without concern for self. There is nothing left to gain.