In order to enter more deeply into the life of prayer and to come to grips with St. Paul's challenge to pray unceasingly, the Orthodox Tradition offers
The Jesus Prayer, which is sometimes called the prayer of the heart.
The Jesus Prayer is offered as a means of concentration, as a focal point for our inner life. Though there are both longer and shorter versions, the most frequently used form of The Jesus Prayer is:
"Lord Jesus Christ, Son of God,
have mercy on me, a sinner."
This prayer, in its simplicity and clarity, is rooted in the Scriptures and the new life granted by the Holy Spirit. It is first and foremost a prayer of the Spirit because of the fact that the prayer addresses Jesus as Lord, Christ and Son of God; and as St. Paul tells us,
"no one can say 'Jesus is Lord'
except by the Holy Spirit"
(1 Cor. 12:3)
Information from the Greek Orthodox Diocese of America LINK