"Lord Jesus Christ, have mercy on me, a sinner." (say 600 times a day)

There are not fixed, invariable rules for those who pray this prayer, "the way there is no mechanical, physical or mental technique which can force God to show his presence" (Metropolitan Kallistos Ware).
People who say the prayer as part of meditation often synchronize it with their breathing; breathing in while calling out to God (Lord Jesus Christ, Son of God) and breathing out while praying for mercy (have mercy on me, a sinner). Another option is to say (orally or mentally) the whole prayer while breathing in and again the whole prayer while breathing out and yet another, to breathe in recite the whole prayer, breathe out while reciting the whole prayer again. One can also hold the breath for a few seconds between breathing in and out. It is advised, in any of these three last cases, that this be done under some kind of spiritual guidance and supervision.
Monks often pray this prayer many hundreds of times each night as part of their private cell vigil ("cell rule"). Under the guidance of an Elder (Russian Starets; GreekGerondas), the monk aims to internalize the prayer, so that he is praying unceasingly. St. Diadochos of Photiki refers in On Spiritual Knowledge and Discrimination to the automatic repetition of the Jesus Prayer, under the influence of the Holy Spirit, even in sleep. This state is regarded as the accomplishment of Saint Paul's exhortation to the Thessalonians to "pray without ceasing" (1 Thessalonians 5:17).
The Jesus Prayer can also be used for a kind of "psychological" self-analysis. According to the "Way of the Pilgrim" account and Mount Athos practitioners of the Jesus Prayer, "one can have some insight on his or her current psychological situation by observing the intonation of the words of the prayer, as they are recited. Which word is stressed most. This self-analysis could reveal to the praying person things about their inner state and feelings, maybe not yet realised, of their unconsciousness."
"While praying the Jesus Prayer, one might notice that sometimes the word “Lord” is pronounced louder, more stressed, than the others, like: LORD Jesus Christ, (Son of God), have mercy on me, (a/the sinner). In this case, they say, it means that our inner self is currently more aware of the fact that Jesus is the Lord, maybe because we need reassurance that he is in control of everything (and our lives too). Other times, the stressed word is “Jesus”: Lord JESUS Christ, (Son of God), have mercy on me, (a/the sinner). In that case, they say, we feel the need to personally appeal more to his human nature, the one that is more likely to understands our human problems and shortcomings, maybe because we are going through tough personal situations. Likewise if the word “Christ” is stressed it could be that we need to appeal to Jesus as Messiah and Mediator, between humans and God the Father, and so on. When the word “Son” is stressed maybe we recognise more Jesus’ relationship with the Father. If “of God” is stressed then we could realise more Jesus’ unity with the Father. A stressed “have mercy on me” shows a specific, or urgent, need for mercy. A stressed “a sinner” (or “the sinner”) could mean that there is a particular current realisation of the sinful human nature or a particular need for forgiveness." "In order to do this kind of self-analysis one should better start reciting the prayer relaxed and naturally for a few minutes – so the observation won’t be consciously “forced”, and then to start paying attention to the intonation as described above. Also, a person might want to consciously stress one of the words of the prayer in particular when one wants to express a conscious feeling of situation. So in times of need stressing the “have mercy” part can be more comforting or more appropriate. In times of failures, the “a sinner” part, etc.…)."

Source: Wikipedia