No unlawful pleasures are compatible with chastity; the pure heart is like the mother of pearl which admits no drop of water save that which comes from Heaven, — it is closed to every attraction save such as are sanctified by holy matrimony. Close your heart to every questionable tenderness or delight, guard against all that is unprofitable though it may be lawful, and strive to avoid unduly fixing your heart even on that which in itself is right and good.
Every one has great need of this virtue: those living in widowhood need a brave chastity not only to forego present and future delights, but to resist the memories of the past, with which a happy married life naturally fills the imagination, softening and weakening the will. Saint Augustine lauds the purity of his beloved Alipius, who had altogether forgotten and despised the carnal pleasures in which his youth was passed. While fruits are whole, you may store them up securely, some in straw, some in sand or amid their own foliage, but once bruised there is no means of preserving them save with sugar or honey. Even so the purity which has never been tampered with may well be preserved to the end, but when once that has ceased to exist nothing can ensure its existence but the genuine devotion, which, as I have often said, is the very honey and sugar of the mind.
The unmarried need a very simple sensitive purity, which will drive away all over-curious thoughts, and teach them to despise all merely sensual satisfactions. The young are apt to imagine that of which they are ignorant to be wondrous sweet, and as the foolish moth hovers around a light, and, persisting in coming too near, perishes in its inquisitive folly, so they perish through their unwise approach to forbidden pleasures. And married people need a watchful purity whereby to keep God ever before them, and to seek all earthly happiness and delight through Him Alone, ever remembering that He has sanctified the state of holy matrimony by making it the type of His own union with the Church.
The Apostle says, "Follow peace with all men, and holiness, without which no man shall see the Lord:" [Heb. 12, 14] by which holiness he means purity. Of a truth, my daughter, without purity no one can ever see God; [Matt. 5,8] nor can any hope to dwell in His tabernacle except he lead an uncorrupt life; [Psalm. 15, 1,2] and our Blessed Lord Himself has promised the special blessing of beholding Him to those that are pure in heart.
How to Maintain Purity
BE exceedingly quick in turning aside from the slightest thing leading to impurity, for it is an evil which approaches stealthily, and in which the very smallest beginnings are apt to grow rapidly. It is always easier to fly from such evils than to cure them.
Human bodies are like glasses, which cannot come into collision without risk of breaking; or to fruits, which, however fresh and ripe, are damaged by pressure. Never permit any one to take any manner of foolish liberty with you, since, although there may be no evil intention, the perfectness of purity is injured thereby.
Purity has its source in the heart, but it is in the body that its material results take shape, and therefore it may be forfeited both by the exterior senses and by the thoughts and desires of the heart. All lack of modesty in seeing, hearing, speaking, smelling, or touching, is impurity, especially when the heart takes pleasure therein. S. Paul says without any hesitation that impurity and uncleanness, or foolish and unseemly talking, are not to be "so much as named" among Christians. The bee not only shuns all carrion, but abhors and flies far from the faintest smell proceeding therefrom. The Bride of the Canticles is represented with "hands dropping with myrrh." a preservative against all corruption; her "lips are like a thread of scarlet," the type of modest words; her eyes are "dove's eyes," clear and soft; her "nose is as the tower of Lebanon which looketh towards Damascus" an incorruptible wood; her ears are hung with earrings of pure gold; and even so the devout soul should be pure, honest and transparent in hand, lip, eye, ear, and the whole body.
Remember that there are things which blemish perfect purity, without being in themselves downright acts of impurity. Anything which tends to lessen its intense sensitiveness, or to cast the slightest shadow over it, is of this nature; and all evil thoughts or foolish acts of levity or heedlessness are as steps towards the most direct breaches of the law of chastity. Avoid the society of persons who are wanting in purity, especially if they are bold, as indeed impure people always are. If a foul animal licks the sweet almond tree its fruit becomes bitter; and so a corrupt pestilential man can scarcely hold communication with others, whether men or women, without damaging their perfect purity — their very glance is venomous, and their breath blighting like the basilisk. On the other hand, seek out good and pure men, read and ponder holy things; for the Word of God is pure, and it will make those pure who study it: wherefore David likens it to gold and precious stones. Always abide close to Jesus Christ Crucified, both spiritually in meditation and actually in Holy Communion; for as all those who sleep upon the plant called Agnus castus become pure and chaste, so, if you rest your heart upon Our Dear Lord, the Very Lamb, Pure and Immaculate, you will find that soon both heart and soul will be purified of all spot or stain.
Excerpts taken from: Introduction to the Devout Life : Chapters XII and XIII