Sacramentals are sacred signs instituted by the Church. They prepare men to receive the fruit of the sacraments and sanctify different circumstances of life. Catechism #1677.

What do they do?
1.Deliver and protect our person and goods from the harmful influences of Satan. Christ gave His Church the power to expel evil spirits. "He gave the power to heal sickness and to cast out devils." (Mk. 3:15). The Church has always made use of this power and hence the means she employs for the casting out of evil spirits, are the sacramentals, such as the invocation of the Holy Name of Jesus, the Sign of the Cross, Holy Water, and in some cases, exorcism, They also protect from diabolical attacks-hence, the Church blesses newly built houses, stables, barns, ships, cars, and other things so that the devil may be kept away from these places and not exercise an injurious effect upon them. Therefore, as well as blessing ourselves with Holy Water and the Sign of the Cross, we should have our houses, cars and other things blessed with the Sacramentals, showing that they are dedicated to God.

2. Avert from evils due to natural causes-they are remedies against all sicknesses, weaknesses and frailties of the body, and are sure to effect relief if this is beneficial for the soul. Sometimes a sickness may be there for God's will for us: "Why do you seek aid from physicians when you are sick, and not from Christ, as if He were not also the physician of your body?" (St Ephrem).

Sacramentals are remedies against epidemics, storms, etc., but only on the condition that the aversion of any evil brings no injury to our salvation. Eg. St. Benedict blessed a chalice of poisoned wine with the Sign of the Cross, and it fell to pieces.

3. Sacramentals remove spiritual evils-can banish sadness, fear and anguish, weaken the passions and help one to overcome temptations-sometimes they cease altogether.

4. Sacramentals can procure for us, natural and supernatural goods.

Natural: Health, prosperity, success in business, bountiful harvest, favorable weather-when the church blesses such things, she delivers them from the curse of sin which has come upon all creation due to the fall. Taking it from the dominion of Satan and restoring it to the state of integrity and goodness which comes from the hand of God. Thus, the Church blesses workplaces and fields etc., for a good harvest!

Supernatural: Sacramentals do not give supernatural grace, but have the virtue of obtaining for the sinner the grace of repentance and conversion. Also, even the remission of venial sins and their temporal punishment. "No sacrament of the New Testament is especially instituted against venial sins, which is taken away by some sacramentals, such as Holy Water and other things." (St. Thomas Aquinas).

On what conditions are Sacramentals effective?
Sacramentals have their power from the prayer of the Church, which is more effectual than prayers of individual, hence they can have an effect upon those who do not participate in them. For example, people who have received the Miraculous Medal of Our Lady, revealed to St. Catherine Laboure, and being converted though they had done nothing more than wear the medal. However,  these are special exceptions, generally he who wishes to partake in the effects of sacramentals must fulfill certain conditions.

1. Have a firm Faith and great confidence.

2. Make use of them with good intentions - that is if they are for example blessing the crop to obtain a good harvest, a temporal good, it must only be desired for the greater glory of God and for the good of one's soul. If the focus was only upon the success of the business without thought for the salvation of one's soul, then the blessing would be an abuse.

"Although it is a great consolation to become well through the instrumentality of pious people and holy means, yet these benefits are not always imparted to those who pray for them, because temporal goods should be sought rather on account of life everlasting, where there are no evils."
(St. Augustine).

3. Be resigned to the will of God - sacramentals have the virtue of removing temporal evils and obtaining temporal benefits. But whether the removal of a temporal evil or granting of a temporal blessing is useful or not is known only to God. A person might use a sacramental for some benefit - free from sickness, but God may know if that cross went away, that person would plunge into sin and be lost. We can make use of sacramentals, whether or not God will make them effectual or not, is according to His will. Our Lord: "Father, not My will, but thine be done."

4. Make use of them with a pure or at least a repentant heart - in the book of Kings we see how the Israelites under the high priest Heli, were defeated by the Philistines, and lost four thousand men. They brought the Ark of the covenant into the campsite  and thought themselves invincible. But they were defeated in a second battle, a greater defeat and lost 30.000 men and the Ark fell into the hands of the Philistines.
(!.Kings 4). Why did this happen? They had the Ark? Because they had sinned and persevered in their sin. God showed no favor to them till they were truly penitent. Likewise, Sacramentals are not beneficial unless we have a pure or at least penitent heart.

Conclusion.  These days there is a tendency upon modern man to dismiss the benefits of Sacramentals such as Holy Water and blessed objects, medals etc., but the Church has always taught the benefits of using Sacramentals. It is important for us to use them, however, under the conditions outlined previously. Respect and honor the Sacramentals, make use of them with faith and confidence and with resignation to the will of God with a pure, or at least, penitent heart. Do this and one may confidently hope to partake in the many graces and blessings which Sacramentals bring.   Amen.

Source: Father Michael Rowe.  Ave Maria Magazine.