Of course, you can always pray and offer sacrifices to help the souls in Purgatory, but here is a more formal way to pray for the deceased. The Marian Helpers offer a Holy Souls Sodality. According to their website,
Our times have become indifferent toward Eternal Life and a righteous Fear of God, consequently a very little awareness of the Souls in Purgatory. Ignorance of the Church as threefold — here on earth as the Church Militant, in purgatory as the Church Suffering, and in heaven as the Church Triumphant — reduces true and holy union and the help each can be for one another. Our responsibility to aid each other here on earth by living holy lives is forsaken and so, too, assistance by Masses and prayers of those suffering in purgatory. The true nature of Mass as the supreme prayer of Jesus to the Eternal Father is lost as many want to sense spiritually good without the sacrifice which makes all things holy.
Few, if any, hear teachings on this basic profession of our Christian Faith as we say – "I believe in…. the communion of saints." It is not only saying words but it is living the unity and bond given us at Baptism. The time to correct this humanistic error is now, we as a vibrant Marian family — the Marian Fathers and the Association of Marian Helpers — have a wholesome challenge as given us by Pope Paul VI in May of 1973:
"While giving help to the souls undergoing purification after completing their earthly journey, teach people that they have (here) no lasting city, but they ought to seek the one that is to come (cf. Heb 13:14). Thus your spiritual and religious life will give you strength for taking on this apostolate, so abundant in fruits, which has been entrusted to you."
We Marian Fathers wish to recommit ourselves, and you our Associate members, to praying daily for the deceased. With this in mind we set forth the Holy Souls Sodality as the fulfillment of our Constitutions #19 "following the abiding faith of the Church in the communion of saints in the mystical Body of Christ, with great devotion honors the memory of the dead and offers suffrages for them."
Find out more at: The Holy Souls Sodality