I just remembered that, like jllsjlls, I too have assembled a glorious collection of thoughts on prayer:
"Haste is the destroyer of devotion.. If we allow ourselves to get into this habit, the interior spirit, which is the source of all merit, becomes dried up, and instead of the highest use of our intellect there is only a lip-worship, and holy thoughts and noble feelings are replaced by a blind and mechanical repetition. Once slave to this habit, it is vain to multiply our words of prayer... The words that rise to our lips mean nothing to our heart and leave no impression on our soul. They are nothing but a useless set of words like those for which our Lord blamed the heathen: When ye pray, speak not much as do the heathen, for they think in their much speaking they may be heard' (Mt. 6:7)" (St. Francis de Sales)
"One of the first things the devil always does is to make people stop praying."
"Satan seeks to disarm you before every temptation. He does this by taking away from you the only instrument that enables you to defeat him: prayer. In prayer, you obtain from God the light of discernment to discover Stan's snares and the strength to oppose his allurements. He will be untiring in his attempt to rob you of your daily moments for prayer, making excuses that there are other more urgent things to do. He will seek to render your prayers insignificant or inconclusive. Once there is no more room for daily prayer, God inevitably disappears from your life. Other things have taken His place, and Satan is in a position to seduce you at his pleasure." (Fr. Fanzaga)
"To my mind there is no labor so great as praying to God: for when a man wishes to pray to his God, the hostile demons make haste to interrupt his prayer, knowing that their sole hindrance is in this, a prayer poured out to God. With any other labor that a man takes in the life of religion, however instant and close he keeps to it, he hath some rest: but prayer hath the travail of a mighty conflict to one's last breath." (St. Agatho)
"In the same way, the Spirit too comes to the aid of our weakness; for we do not know how to pray as we ought, but the Spirit itself intercedes with inexpressible groanings. And the one who searches hearts knows what is the intention of the Spirit, because it intercedes for the holy ones according to God's will." (St. Paul, Rom. 8:26-27)
"Do not be either astonished or discouraged at the difficulties you find in prayer. Only be constant and submissive and God will be pleased with you." (St. Claude de la Colombiere)
"Prayer is nothing else than speaking to God; and to speak to God without concentrating our attention upon Him is a thing most odious to His Divine Majesty." (St. Francis de Sales, Doctor of the Church)
"He tempts God who prays well but acts badly, and while he converses with God allows his mind to wander." (Catechism of the Council of Trent)
"In sickness itself, or amidst casual happenings, so long as the soul is a loving one it is always possible to pray by offering up the distraction itself and remembering Him for whom we are suffering it." (St. Teresa of Avila, Doctor of the Church)
"How can you ask to be heard by God when you yourself aren't even paying attention to what you say? How can you want God to remember you when you pray if you yourself don't even remember what you say?" (St. Cyprian of Carthage)
"If you find it impossible to pray, hide behind your good angel and charge him to pray in your stead." (St. John Vianney)
"If someone, simply as an experiment, should try with determination to make his mind touch on as many and as diverse objects as possible, I hardly think that in so short a time he could run through such diverse and numerous topics as the mind, unrestrained, wanders through while the mouth negligently mutters through the most common prayers." (St. Thomas More)
"Never give up prayer, and should you find dryness and difficulty, persevere in it for this very reason. God often desires to see what love your soul has, and love is not tried by ease and satisfaction." (St. John of the Cross, Doctor of the Church)
"To my mind there is no labor so great as praying to God: for when a man wishes to pray to his God, the hostile demons make haste to interrupt his prayer, knowing that their sole hindrance is in this, a prayer poured out to God. With any other labor that a man takes in the life of religion, however instant and close he keeps to it, he hath some rest: but prayer hath the travail of a mighty conflict to one's last breath." (St. Agatho)
"Not only pleasure will withdraw men from prayer, but also affliction sometimes; but there is this difference: affliction will sometimes exhort a short prayer from the wickedest man alive, but pleasure stifles it altogether." (St. Thomas More)
"What I only aim at is this: that we see Him, and remain with Him, to whom we are speaking, without turning our backs on Him. For I think that is what we do when we keep talking to God while thinking about a thousand trifles at the same time." (St. Teresa of Avila, Doctor of the Church)
"...neglect of prayer, lukewarmness in its performance and the omission of prescribed prayers, are always a step in advance on the way to perdition." (Fr. Groenings)
"Even holy men sometimes suffer from a wandering of the mind when they pray, according to Psalm 40:12, 'My heart hath forsaken me.'" (St. Thomas Aquinas, Doctor of the Church and "greatest theologian in the history of the Church")
"[T]he more violently we are assailed by our restless thoughts, the more fervently ought we to give ourselves to prayer." (St. Gregory of nαzιanzus, Doctor of the Church)
"What insensibility is it to be snatched wandering off by light and profane imaginings, when you are presenting your entreaty to the Lord as if there were anything else you ought rather to consider than that your converse is with God! How can you claim of God to attend to you, when you do not attend to yourself? This is altogether to make no provision against the enemy; this is when praying to God, to offend God's Majesty by the neglectfulness of your prayer." (St. Cyprian)
"A friar said to Brother Giles: 'It is written of St. Bernard that he once said the seven penitential psalms and did not think of anything but what he was saying.' The holy Brother Giles answered: 'I think it is a greater thing if an armed camp is strongly attacked, and the sentinel of the camp defends himself manfully and bravely.'"
"The human mind is unable to remain aloft for long on account of the weakness of nature, because human weakness weighs down the soul to the level of inferior things: and hence it is that when, while praying, the mind ascends to God by contemplation, of a sudden it wanders off through weakness... Purposely to allow one's mind to wander in prayer is sinful and hinders the prayer from having fruit. It is against this that [St.] Augustine says in his Rule (Ep. 211): 'When you pray God with psalms and hymns, let your mind attend to that which your lips pronounce.' But to wander in mind unintentionally does not deprive prayer of its fruit. Hence Basil says (De Constitutione Monachorum i): 'If you are so truly weakened by sin that you are unable to pray attentively, strive as much as you can to curb yourself, and God will pardon you, seeing that you are unable to stand in His presence in a becoming manner, not through negligence but through frailty.'" (St. Thomas Aquinas, Doctor of the Church and "greatest theologian in the history of the Church")
"Then a friar asked [Br. Giles]: ' What can I do to go willingly into prayer when I feel dry and lacking in devotion?' He replied: 'Suppose a king has two servants, one of whom is armed, but the other is unarmed, and they have to go to war. The one who is armed goes bravely to war, but the other who is unarmed says this to his lord: 'My lord, as you see, I have no weapons. But because I love you I will go into battle even without weapons.' Now the king on seeing the faithfulness of that servant says to his attendants: 'Go and prepare armor to adorn this faithful servant of mine, and place on him the emblem of my own armor.' So too if someone goes into the battle of prayer is if without arms, because he feels dry and lacking devotion, God sees his faithfulness and paces the emblem of His armor on him."
"Sometimes, also, such is the remissness and negligence with which we pray, that we ourselves do not attend to what we say. Since prayer is an elevation of the soul to God, if, while we pray, the mind, instead of being fixed upon God, is distracted, and the tongue slurs over the words at random, without attention, without devotion, with what propriety can we give to such empty sounds as the name of Christian prayer? We should not, therefore, be at all surprised, if God does not comply with our requests; either because of or negligence and indifference with almost show that we do not really desire what we ask, or because we ask those things, which, if granted, would be prejudicial to our interests." (Catechism of the Council of Trent)
"A friar once asked [Brother Giles]: 'Why does a man suffer more temptations when he is praying to God than at other times?' The holy Brother Giles answered him: 'When someone has a lawsuit against some opponent in the court of a prince, if that man goes to the prince to suggest that something be done against his opponent, the latter on hearing about it objects with all his power so that the decision should not be given in favor of the former. That is the way the devil acts against us. For if you spend time in conversation with others, you will often see that you do not feel many attacks of temptation. But if you go to pray and refresh your soul, then you will feel the burning arrows of the enemy against you. However, you should not give up prayer on that account, but you should stand firm. Because that is the way to our home above, and he who gives up prayer because of it is like a man who runs away from battle."