The Jesus Prayer is a treasure of the Eastern Church and many saints have written on it. There are several variations of the Jesus Prayer. Its most common form is:
Lord Jesus Christ, Son of God, have mercy on me!
St. John Chrysostom wrote the following instruction on the value of the Jesus Prayer:
…constantly call: “Lord, Jesus Christ, Son of God, have mercy upon me!” in order that this remembrance of the Name of our Lord Jesus should incite you to battle with the enemy. By this remembrance, a soul forcing itself to do this practice can discover everything which is within, both good and bad. First, it will see within, in the heart, what is bad, and later — what is good. This remembrance is for rousing the serpent, and the remembrance is for subduing it. This remembrance can reveal the sin living in us, and this remembrance can destroy it. This remembrance can arouse all the enemy hosts in the heart, and little by little this remembrance can conquer and uproot them.
The Name of our Lord Jesus Christ, descending into the depths of the heart, will subdue the serpent holding sway over the pastures of the heart, and will save our soul and bring it to life. Thus abide constantly with the Name of our Lord Jesus Christ, so that the heart swallows the Lord and the Lord the heart, and the two become one. But this work is not done in one or two days; it needs many years and a long time. For great and prolonged labor is needed to cast out the foe so that Christ dwells in us….
For this great work demands great forcing, since “straight is the gate and narrow is the way that leads to life” (Matthew 7:14). And only those who force themselves enter the Kingdom of Heaven, for the “violent take it by force” (Matthew 11:12). I implore you therefore not to withdraw your hearts from God, but to watch them and guard them by constant remembrance of our Lord Jesus Christ, until the name of our Lord Jesus Christ is deeply rooted in your heart and you cease to think of aught but glorifying the Lord in you. — From The Publicans Prayer Book, pp. 564-565.