Origen the Liturgist

Origen's treatise On Prayer has an interesting section at the end about the formalities of prayer which give an interesting insight into 3rd century practices.
On standing and praying

Of all the innumerable dispositions of the body that, accompanied by outstretching of the hands and upraising of the eyes, standing is preferred—inasmuch as one thereby wears in the body also the image of the devotional characteristics that become the soul. I say that these things ought to be observed by preference except in any special circumstances, for in special circumstances, by reason of some serious foot disease one may upon occasion quite properly pray sitting, or by reason of fevers or similar illnesses, lying, and indeed owing to circumstances, if, let us say, we are on a voyage or if our business does not permit us to retire to pay our debt of prayer, we may pray without any outward sign of doing so.

But kneeling has its place as well

Moreover, one must know that kneeling is necessary when he is about to arraign his personal sins against God with supplication for their healing and forgiveness, because it is a symbol of submission and subjection. For Paul says; For this cause I bow my knees unto the Father from whom is all fatherhood named in heaven and on earth. It may be termed spiritual kneeling, because of the submission and self-humiliation of every being to God in the name of Jesus, that the apostle appears to indicate in the words: that in the name of Jesus every knee should bow in heaven and on earth and under the earth.
It should not be supposed that beings in heaven have bodies so fashioned as actually to possess knees, since their bodies have been described possibly as spherical in form by those who have discussed these matters more minutely. He who refuses to admit this will also, unless he outrages reason, admit the uses of each of the members in order that nothing fashioned for them by God may be in vain. One falls into error on either hand, whether he shall assert that bodily members have been brought into being by God for them in vain and not for their proper work, or shall say that the internal organs, the intestine included, perform their proper uses even in heavenly beings. Exceedingly foolish will it be to think that it is only their surface, as with statues, that is human in form and nothing further underneath.

A very literal ad orientem

A few words may now be added in reference to the direction in which one ought to look in prayer. Of the four directions, the North, South, East, and West, who would not at once admit that the East clearly indicates the duty of praying with the face turned towards it with the symbolic suggestion that the soul is looking upon the dawn of the true light?
Should anyone, however, prefer to direct his intercessions according to the aperture of the house, whichever way the doors of the house may face, saying that the sight of heaven appeals to one with a certain attraction greater than the view of the wall, and the eastward part of the house having no opening, we may say to him that since it is by human arrangement that houses are open in this or that direction but by nature that the East is preferred to all the other directions, the natural is to be set before the artificial. Besides, on that view why should one who wished to pray when in the open country pray to the East in preference to the West? If, in the one case it is reasonable to prefer the East, why should the same not be done in every case? Enough on that subject.

On the Holy Spirit

From the Parochial & Plain Sermons:

The Holy Ghost, I have said, dwells in body and soul, as in a temple. Evil spirits indeed have power to possess sinners, but His indwelling is far more perfect; for He is all-knowing and omnipresent, He is able to search into all our thoughts, and penetrate into every motive of the heart. Therefore, He pervades us (if it may be so said) as light pervades a building, or as a sweet perfume the folds of some honourable robe; so that, in Scripture language, we are said to be in Him, and He in us. It is plain that such an inhabitation brings the Christian into a state altogether new and marvellous, far above the possession of mere gifts, exalts him inconceivably in the scale of beings, and gives him a place and an office which he had not before. In St. Peter's forcible language, he becomes "partaker of the Divine Nature," and has "power" or authority, as St. John says, "to become the son of God." Or, to use the words of St. Paul, "he is a new creation; old things are passed away, behold all things are become new." His rank is new; his parentage and service new. He is "of God," and "is not his own," "a vessel unto honour, sanctified and meet for the Master's use, and prepared unto every good work." [2 Pet. i. 4. John i. 12. 2 Cor. v. 17. 1 John iv. 4. 1 Cor. vi. 19, 20. 2 Tim. ii. 21.]

This wonderful change from darkness to light, through the entrance of the Spirit into the soul, is called Regeneration, or the New Birth; a blessing which, before Christ's coming, not even Prophets and righteous men possessed, but which is now conveyed to all men freely through the Sacrament of Baptism. By nature we are children of wrath; the heart is sold under sin, possessed by evil spirits; and inherits death as its eternal portion. But by the coming of the Holy Ghost, all guilt and pollution are burned away as by fire, the devil is driven forth, sin, original and actual, is forgiven, and the whole man is consecrated to God. And this is the reason why He is called "the earnest" of that Saviour who died for us, and will one day give us the fulness of His own presence in heaven. Hence, too, He is our "seal unto the day of redemption;" for as the potter moulds the clay, so He impresses the Divine image on us members of the household of God. And His work may truly be called Regeneration; for though the original nature of the soul is not destroyed, yet its past transgressions are pardoned once and for ever, and its source of evil staunched and gradually dried up by the pervading health and purity which has set up its abode in it. {224} Instead of its own bitter waters, a spring of health and salvation is brought within it; not the mere streams of that fountain, "clear as crystal," which is before the Throne of God, but, as our Lord says, "a well of water in him," in a man's heart, "springing up into everlasting life." Hence He elsewhere describes the heart as giving forth, not receiving, the streams of grace: "Out of his belly shall flow rivers of Living Water." St. John adds, "this spake He of the Spirit." [John iv. 14; vii. 38, 39.]

V - Come, Holy Spirit, fill the hearts of your faithful.
R - And kindle in them the fire of your love.

V - Send forth your spirit, Lord, and they shall be created.
R - And thou shalt renew the face of the earth.

Let us pray,
O God, who by the light of the Holy Spirit did instruct the hearts of your faithful; grant that in the same Spirit we may be truly wise and ever rejoice in his consolation. Through the same Christ, our Lord. Amen.

Prayer of St Thomas Aquinas on Receiving Viaticum

If in this world there be any knowledge of this sacrament stronger than that of faith, I wish now to use it in affirming that I firmly believe and know as certain that Jesus Christ, True God and True Man, Son of God and Son of the Virgin Mary, is in this Sacrament.
I receive Thee, the price of my redemption, for Whose love I have watched, studied, and laboured. Thee have I preached; Thee have I taught. Never have I said anything against Thee: if anything was not well said, that is to be attributed to my ignorance. Neither do I wish to be obstinate in my opinions, but if I have written anything erroneous concerning this sacrament or other matters, I submit all to the judgement and correction of the Holy Roman Church, in whose obedience I now pass from this life.