As the previous posts have indicated we can easily distracted in prayer by visions, feelings of warmth and joy. Because of this it is not always appropriate for one to practice the Jesus Prayer. First there is preparation that is needed.
Saint Isaac the Syrian says,
God's grace comes of itself without any ambitious striving on our part. It will only come to the heart that is pure.... Should the apple of thine eye be unclean, dare not to raise it; attempt not to gaze at the ball of the sun; lest they temerity deprive thee even of the limited sight, acquired through simple faith, humility, penance, and other lowly acts and works; lest thy temerity be punished and thou fall headlong into the outer darkness.Elder Macarius writes in one of his letters the following:
It was a mistake for you to practice mental prayer and prayer of the heart. All this is beyond your strength, outside the scope of your capacities, incompatible with your circumstances. Such practices exact the strictest purity of intention towards God, men, and even things.
Instead, read or recite––under the direction of your confessor––psalms, penitential canons, litanies and so on. Go to church as frequently as possible; live humbly, according to the admonitions of your conscience; and carefully, according to the commandments of our Lord. In other words, lead the life of an ordinary, God-fearing member of the Christian laity.Callistus and Ignatius write,
Many paths may lead either to salvation or perdition. But one there is which securely leads us heavenward: a life lived according to our Lord's commandments.Our task is to "constantly practice humility, love and charity" elder Macarius reminds us.
Reference: Russian Letters of Spiritual Direction, p 107,108, 110