Let us start with St. John Chrysostom (the Golden-mouthed): before he died of tuberculosis while traveling in exile, his last words were “Glory be to God for all things.”
He lived and died according to his preaching, as he wrote, in his homilies gathered in the Patrologia Græca: “Even if it be disease or poverty, for seen and unseen benefit, and for those which we receive against our will; but also whenever we are either in poverty, or in sicknesses, or are being insulted, then let us intensify our thanksgiving; thanksgiving, I mean, not in words, nor with the tongue, but in deeds and works, in mind and in heart; let us give thanks to Him with all our souls.” 
“Have you fallen seriously ill? This brings you the crown of martyrdom [through thanksgiving]. Nothing is holier than that tongue which gives thanks to God in evil circumstances; truly in no respect does it fall short of that of Martyrs; both alike are crowned, both the former and the latter”.

On the Eucharist (from the ecclesiastical Greek eukharistia ‘thanksgiving,’ from Greek eukharistos ‘grateful,’ from eu ‘well’ + kharizesthai ‘offer graciously’, from kharis ‘grace’), St. John Chrysostom said: "The dread Mysteries, full of such great salvation, which are celebrated at every Liturgy, are also called a Thanksgiving [Eucharistia] because they are the remembrance of many benefits, and they signify the culmination of God’s Providence towards us, and in every way cause us to be thankful to Him." 

How are we to preserve our blessings?

"Let us give thanks to God continually. For, it is outrageous that when we enjoy His benefaction to us in deed every single day, we do not acknowledge the favor with so much as a word; and this, when the acknowledgment confers great benefit on us. He does not need anything of ours, but we stand in need of all things from Him.
In point of fact, thanksgiving adds nothing to Him, but it brings us closer to Him. For if, when we recall the benefactions of men, we are the more warmed by affection for them; much more, when we continually bring to mind the benefits of the Master towards us, shall we be more earnest with regard to His commandments.
For this cause Paul also said, Be ye thankful. For the best preservative of any benefaction is the remembrance of the benefaction, and a continual thanksgiving for it." 

What shall I render unto the Lord for all that He has rendered unto me?

"He brought us from non-being in being; He dignified us with reason; He provided us with crafts to help sustain our lives; He causes food to spring up from the earth; He has given us cattle to serve us. For our sake there is rain, for our sake there is the sun; the hills and plains have been adorned for our benefit, affording us refuge from the peaks of the mountains. For our sake rivers flow; for our sake fountains gush forth; the sea is made calm for our trading; riches come from mines and delights from everywhere, and the whole of creation is offered as a gift to us, on account of the rich and abundant Grace of our Benefactor towards us.
But why speak of minor gifts? For our sake God lived among men; for the sake of our corrupt flesh, the Word was made flesh, and dwelt among us. To the thankless He was their Benefactor; to those sitting in darkness, the Sun of Righteousness; upon the Cross He was the Impassible One; in death, the Life; in Hades, the Light; the Resurrection for the fallen; the spirit of adoption into sonship, bestowals of spiritual gifts, and promises of crowns.
In addition to such great and splendid benefits, or rather, benefits par excellence, the benefits that He promises us in the future life are many times greater: the delight of Paradise, glory in the Kingdom of Heaven, honors equal to those of the Angels, and the vision of God, which, for those counted worthy of it, is the highest of all goods; every rational nature desires this, and may we also attain to it, after we have cleansed ourselves of carnal passions." 

St. John of Sinai, in The Ladder of Divine Ascent, also had something to add: "Before all else, let us list sincere thanksgiving first on the scroll of our prayer. On the second line, we should put confession and heartfelt contrition of soul. Then let us present our petition to the King of all. This is the best way of prayer, as it was shown to one of the brethren by an angel of the Lord." 

“I think, said St. Gregory of Nyssa, that if our whole life long we conversed with God without distraction, and did nothing but give thanks, we should really be just as far from adequately thanking our heavenly Benefactor as if we had never thought of thanking Him at all. For time has three parts — the past, the present, and the future. If you look at the present, it is by God that you are now living; if the future, He is the hope of everything you expect; if the past, you would never have been if He had not created you. That you were born was His blessing; and after you were born, your life and your death were, as the apostle Paul says, equally His blessing. Whatever your future hopes may be, they hang also upon His blessing. You are master only of the present, and therefore, if you never once intermitted thanksgiving during your whole life, you would hardly do enough for the grace that is always present; and your imagination cannot conceive of any method possible, by which you could do anything for the time past, or for the time to come."

According to St. Peter of Damaskos, whose writings can be found in the Philokalia - A Treasury of Divine Knowledge, "Freedom from anxiety makes it (the heart) rejoice and give thanks; the grateful offering of thanks augments the gifts of grace it has received. And as the blessings increase, so does the thankfulness, and so does the pure prayer offered with tears of joy. Slowly the man emerges from the tears of distress and from the passions and enters fully into the state of spiritual joy." Also, "he who has received a gift from God, and is ungrateful for it, is already on the way to losing it." And finally, "when God is thanked, He gives us still further blessings, while we, by receiving His gifts, love Him all the more and through this love attain that divine wisdom whose beginning is the fear of God." 

For John the Solitary, quoted in The Syriac Fathers on Prayer and the Spiritual Life, "When evening comes, collect your thoughts and ponder over the entire course of the day: observe God’s providential care for you; consider the grace He has wrought in you throughout the whole span of the day; consider the rising of the moon, the joy of daylight, all the hours and moments, the divisions of time, the sight of different colors, the beautiful adornment of creation, the course of the sun, the growth of your own stature, how your own person has been protected, consider the blowing of the winds, the ripe and varied fruits, how the elements minister to your comfort, how you have been preserved from accidents, and all the other activities of grace. When you have pondered on all this, wonder of God’s love toward you will well up within you, and gratitude for his acts of grace will bubble up inside you."

A small mention of the sin or ingratitude with St. Gregory the Wonderworker to close today's topic, "Ingratitude is despicable…the most despicable thing of all. For someone who has experienced something good not to try to return the favor, even if he can manage no more than verbal thanks, he must plainly be obtuse and insensitive to his benefits, or thoughtless.”

Picture: 

Thank you - CC0 Public Domain.