The “Jesus Prayer” is the most powerful prayer, which contains the words “Lord Jesus Christ, have mercy of me.” This core original, traditional formula varied and varies slightly, depending on the historical period and the teaching school which used it. The most common additions, but not the only ones, are “Lord Jesus Christ, [son of God], have mercy on me, [the sinner].” And there are other variations.
In any case, we recommend the shortest form because, especially nowadays, our minds are very weak—so the shorter, original, form will help the mind keep the prayer more easily.
The golden rule is to not form any kind of thoughts, and to throw away any thoughts and images which come. We should just be attentive at the words of the prayer and close our minds inside them.
One should say it as much as possible—more or less unceasingly—preferably with the help of a prayer rope because it gathers the mind much better.
At the beginning, it is better to say the prayer with your mouth, whispering (if you don’t distract others). Whispering the prayer helps a lot, especially when someone does manual tasks.
Of course, you must have a normal spiritual life in church—regular confession being the most important aspect here. Another very important aspect from our spiritual life is obedience. Being disobedient is a sure way to fill one’s mind with a lot of thoughts and demonic war which will greatly hinder prayer.
“Noetic prayer” (also called “the prayer of the mind”) and “the prayer of the heart” are just two special stages of the Jesus prayer.
God willing, if a person advances in prayer, then the prayer starts to “work”—that is to say, work alone, in the person’s “mind.” The person doesn’t make any effort anymore, but the prayer is said unceasingly by God’s grace, even if the person is speaking, eating, or even sleeping. When we say that the prayer is said in a person’s “mind”—that is, the “prayer of the mind” or “noetic prayer,” we say that the prayer is said in the rational aspects (parts) of the soul that is in the areas of logic and intellect (νούς), usually translated as “mind.”
If a person advances more, then God’s grace takes the prayer from the mind into the “heart”—that is, into the irrational aspects of the soul—the inflammatory aspect and the appetitive aspect.
Then, the “mind” and the “heart” become united—and the person will overcome the internal chaotic behavior of his/her aspects, and will then reach a deep, peaceful power: joy and wisdom conferred by the Grace in the unity of their soul.
If we will not do the prayer well, it will not have the desired results, which is, of course, undesirable. But do not think that it will have adverse effects—except in cases in which someone does the prayer while having thoughts of big pride, infatuation, delusion, seeking miracles, lights, angels, etc.
Falling in delusion or having neurotic problems is the classical outcome of a very self-confident, proud soul. If someone confesses regularly, does not believe and does not give importance to his thoughts and various ‘pseudo-spiritual experiences (which the evil one might bring), he is very safe. Do not be afraid to pray the Jesus prayer because of this.