How to Say the Prayer
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 (cf. 1 Cor. 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.
St Gregory of SinaiOn Prayer: Seven Texts