Concerning the Jesus Prayer
From the Jordanville Prayer Book (1996 ed.)
In the First Epistle to the Thessalonians the Apostle Paul says: Pray without ceasing. How, then, is one to pray unceasingly? By often repeating the Jesus Prayer: "Lord Jesus Christ, Son of God, have mercy on me." If one becomes accustomed to this appeal, great consolation and the need to continually make this petition will be felt within, and it will be carried on, as if of itself, within one.
Although in the beginning the enemy of the human race will offer hindrances to this, by causing great weariness, indolence, boredom, and over-powering sleep, having withstood all these with the help of God, one will receive peace of soul, spiritual joy, a benevolent disposition towards people, tranquility of thought, and gratitude toward God. In the very name of Jesus Christ a great and graceful power is inherent. Many holy and righteous people advise how one can often, almost without interruption, perform the Jesus Prayer.
Saint John Chrysostom says: "It is necessary for everyone, whether eating, drinking, sitting, serving, traveling, or doing anything, to unceasingly cry: 'Lord Jesus Christ, Son of God, have mercy on me,' that the name of the Lord Jesus Christ, descending into the depths of the heart, may subdue the pernicious serpent, and save and quicken the soul.
Saint Seraphim of Sarov: "Lord Jesus Christ, Son of God, have mercy on me a sinner": let all thine attention and training be in this. Walking, sitting, doing, and standing in church before the divine service, coming in and going out, keep this unceasingly on thy lips and in thy heart. In calling in this manner on the name of God thou wilt find peace, thou wilt attain to purity of spirit and body, and the Holy Spirit, the Origin of all good things, will dwell in thee, and He will guide thee unto holiness, unto all piety and purity."
Bishop Theophanes the Recluse: "In order to more conveniently become accustomed to the remembrance of God, for this the fervent Christian has a special means, namely, to repeat unceasingly a brief prayer of two or three words. Most often this is: 'Lord, have mercy!' or 'Lord Jesus Christ, have mercy on me a sinner.' If you have not yet heard of this, then hear it now, and if you have not done it, then begin to do it from this time.
"Those who have truly decided to serve the Lord God must train themselves in the remembrance of God and in unceasing prayer to Jesus Christ, saying mentally: 'Lord Jesus Christ, Son of God, have mercy on me a sinner.'
"Through such practice, by guarding oneself from distraction and by the preservation of the peace of one's conscience, it is possible to draw near to God and to be united with Him. For, according to the words of Saint Isaac the Syrian, 'Without unceasing prayer we cannot draw near to God' (St. Seraphim of Sarov)."
Saint John of Kronstadt likewise frequently counseled the doing of the Jesus Prayer.
For more information see the outstanding Jesus Prayer Library.