The Tenth Commandment is: Thou shalt not covet thy neighbors goods. It forbids us to desire to rob, cheat or injure another in his property. Just as it is possible to sin by desire against the virtue of purity, so too, it is possible to sin by desire against justice. If a person plans in his own mind how he may rob or cheat or injure his neighbor in his property, if it were in his power to do so, he is already guilty of the sin of injustice in God's sight, even though he may never lay a hand on his neighbors goods.

These two Commandments (the Ninth and the Tenth) remind us that all good and all evil are first conceived in the heart; and hence it is necessary for us to keep our hearts pure, free from all evil desires. "A good man." says Jesus Christ, "out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil." (Luke 5:45).

We should  as followers of Jesus Christ first empty our  hearts of all wickedness: "Blessed are the clean of heart for they shall see God." (Matt. 5:8). And secondly, we should ask Him to fill them with His love: "Heart of Jesus burning with love for me, inflame my heart with love for Thee."

Source: Rev. W. Frean, Commentary on the Catechism.

St John says that "all that is in the world is the concupiscence of the flesh, the concupiscence of the eyes, and the pride of life." Now all that is desirable is included in these three, two of which are forbidden by the precept: "Thou shalt not covet thy neighbors house." (The text of Exod. 20:17 which contains the Ninth and Tenth Commandment reads as follows: "Thou shalt not covet thy neighbors house; neither shalt thou desire his wife, nor his servant, nor his handmaid, nor his ox, nor his ass, nor anything that is his." Here house, signifying "stature," refers to avarice, for "glory and wealth shall be in his house." (Ps. 111:3). This means that he who desires his neighbors house, desires honors and riches. Thus, after the precept of forbidding desire for the house of one's  neighbor, comes the Commandment prohibiting concupiscence of the flesh: Thou shalt not covet thy neighbors wife.

Occasions when sin rules in the flesh
Because of the corruption which resulted from the Fall, none have been free from this concupiscence except Christ and the glorious Virgin.Yet, wherever there is concupiscence, there is either venial or mortal sin - if it is allowed to dominate the reason. When concupiscence reigns in the heart (by our consenting to it), sin rules in the flesh. Therefore St.Paul adds "Let not sin reign in your mortal body." (Rom. 6:12). Accordingly the Lord says "Whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart." Matt 5:28). For with God, the intention is taken for the act.

How concupiscence can be over come
We must realize that the avoidance of concupiscence demands much labor, for concupiscence is grounded in something within us. It is as hard as trying to capture an enemy in one's household. However, there are ways to overcome concupiscence.
By fleeing its external occasions (such as, for instance, bad company and whatever else may be an occasion of sin).
By mortification of the flesh and not by giving an opening to thoughts which of themselves are the occasion of lustful desires.
By perseverance in prayer, concupiscence can be over come: "This kind is not cast out save by prayer and fasting."(Mark 9:29).
By keeping oneself busy with wholesome occupations. "Idleness hath taught much evil." (Ecclus. 33:29).

Between the spirit and the flesh there is a continual combat. If you wish the spirit to win, you must assist it by prayer and resist the flesh by such means as fasting, for by fasting the flesh is weakened. Also St. Jerome says: "Be always busy in doing something good, so that the Devil may find you ever occupied, love to study the Scriptures and you will not love the vices of the flesh."

Source: St. Thomas Aquinas, God's Greatest Gifts.