Sometimes, and most often, you should sit on a stool, because it is more arduous; but sometimes, for a break, you should sit for a while on a mattress. As you sit be patient and assiduous, in accordance with St. Paul's precept, "Cleave patiently to prayer" (Colossians 4:2). Do not grow discouraged and quickly rise up again because of the strain and effort needed to keep your intellect concentrated on its inner invocation. It is as the prophet says: "The birth-pangs are upon me, like those of a woman in travail" (Isaiah 21:3). You must bend down and gather your intellect into your heart -- provided it has been opened -- and call on the Lord Jesus to help you. Should you feel pain in your shoulders or in your head -- as you often will -- endure it patiently and fervently, seeking the Lord in your heart. For "the kingdom of God is entered forcibly, and those who force themselves take possession of it" (Matthew 11:12). With these words the Lord truly indicated the persistence and labor needed in this task. Patience and endurance in all things involve hardship in both body and soul.
-- Some of the fathers advise us to say the whole prayer, "Lord Jesus Christ, Son of God, have mercy," while others specify that we say it in two parts -- "Lord Jesus Christ, have mercy," and then "Son of God, help me" -- because this is easier, given the immaturity and feebleness of our intellect. For no one on his own account and without the help of the Spirit can mystically invoke the Lord Jesus, for this can be done with purity and in its fullness only with the help of the Holy Spirit (I Corinthians 12:3). Like children who can still speak only falteringly, we are unable by ourselves to articulate the prayer properly. Yet we must not out of laziness frequently change the words of the invocation, but only do this rarely, so as to ensure continuity. Again, some fathers teach that the prayer should be said aloud; others, that it should be said silently with the intellect. On the basis of my personal experience I recommend both ways. For at times the intellect grows listless and cannot repeat the prayer, while at other times the same thing happens to the voice. Thus we should pray both vocally and in the intellect. But when we pray vocally we should speak quietly and calmly and not loudly, so that the voice does not disturb and hinder the intellect's consciousness and concentration. This is always a danger until the intellect grows accustomed to its work, makes progress and receives power from the Spirit to pray firmly and with complete attention. Then there will be no need to pray aloud -- indeed, it will be impossible, for we shall be content to carry out the whole work with the intellect alone.
from The Philokalia: Volume IV, edited and translated by G. E. H. Palmer, Philip Sherrard, and Bishop Kallistos Ware, (London: Faber and Faber, 1995), pp. 275 - 276.