By Elder John (Alexeev) of Valaam
It is very sad to hear that the priests teach their spiritual children to imagine the Savior, or the Mother of God, or some Saint in the mind while praying.
This method of prayer is wrong, even harmful. I know that the ones who prayed so, became of unsound mind and went to the doctors to be treated.
I'll tell you briefly how to pray according to the divinely wise Holy Fathers. The mind must be enclosed in the words of the prayer and attention should be kept in the top of the chest, for attention is the soul of the prayer. You should not push the heart with attention, since if there is attention in the chest, the heart will sympathize. When tenderness and warmth of heart appear, do not think that you have received something great. It happens naturally because of concentration, but it is not delusion. Yet God gives some consolation to the praying person by grace.
By all means try not to judge anyone for anything. What you do not want for yourself, do not do to others, nor bear hatred, or the prayer will not become established in the heart.
Source: Letters of the Valaam Elder Schema-Abbot John (Alexeev); January 17, 1954