Today's gospel from Luke 18 was the story of the Tax Collector and the Pharisee. The tax collector , who was justified by his humility in the face of God, prayed, "O God, be merciful to me, a sinner."
This put me in mind of the Eastern Christian tradition of the Jesus Prayer. This prayer is said in groups of 33 or commonly, 100, and kept track of on a prayer rope or chotki. The Jesus Prayer is also called The Prayer of the Heart.
It is suggested that you pray the prayer to the rhythm of your breathing, praying "Lord Jesus Christ, Son of God," as you inhale and "Have mercy on me, a sinner" as you exhale. As it becomes a part of your day, it can be a way of following Paul's directive in Thessalonians to "Pray without ceasing." (1 Thessalonians 5:17
Other prayers that can open or close the Byzantine Prayer Rope are the Prayer of the Righteous Thief "Jesus, remember me when You come into Your Kingdom" or Psalm 51.
Here is my YouTube version of Psalm 51, set to Byzantine chant from the Great Friday Liturgy.
For more information see this site. To purchase one, see the St. Seraphim Prayer Rope page.