31 Moreover, when we stand praying, beloved brethren, we ought to be watchful and earnest with our whole heart, intent on our prayers. Let all carnal and worldly thoughts pass away, nor let the soul at that time think on anything but the object only of its prayer. For this reason also the priest, by way of preface before his prayer, prepares the minds of the brethren by saying, “Lift up your hearts,” that so upon the people's response, “We lift them up unto the Lord,” he may be reminded that he himself ought to think of nothing but the Lord. Let the breast be closed against the adversary, and be open to God alone; nor let it suffer God's enemy to approach to it at the time of prayer.
For frequently he steals upon us, and penetrates within, and by crafty deceit calls away our prayers from God, that we may have one thing in our heart and another in our voice, when not the sound of the voice, but the soul and mind, ought to be praying to the Lord with a simple intention. But what carelessness it is, to be distracted and carried away by foolish and profane thoughts when you are praying to the Lord, as if there were anything which you should rather be thinking of than that you are speaking with God!
How can you ask to be heard of God, when you yourself do not hear yourself? Do you wish that God should remember you when you ask, if you yourself do not remember yourself? This is absolutely to take no precaution against the enemy; this is, when you pray to God, to offend the majesty of God by the carelessness of your prayer; this is to be watchful with your eyes, and to be asleep with your heart, while the Christian, even though he is asleep with his eyes, ought to be awake with his heart, as it is written in the person of the Church speaking in the Song of Songs, I sleep, yet my heart wakes. Wherefore the apostle anxiously and carefully warns us, saying, “Continue in prayer, and watch in the same;” teaching, that is, and showing that those are able to obtain from God what they ask, whom God sees to be watchful in their prayer.
Source: The Treatises of Cyprian (New Advent)