Tradition states that the following was given to St. Gertrude the great. One version says that it will release One Thousands Souls each time it is prayed. As it has not been officially recognized as an indulgenced prayer, we cannot support that claim. But, certainly it will help the poor souls.
Eternal Father, I offer Thee the Most Precious Blood of Thy Divine Son, Jesus, in union with the masses said throughout the world today, for all the holy souls in purgatory, for sinners everywhere, for sinners in the universal church, those in my own home and within my family. Amen.
The following is found in the Pieta prayerbook
There are more souls released from Purgatory during the Consecration of the Mass than at any time. Christmas is the day of the year when the most souls are delivered; then feast days of the Lord, Our Lady and great saints. Souls receive much grace from prayers offered for them on their birthdays, day of Baptism, anniversary of death.
The more we work for Poor Souls on earth, the more others will pray for us, and the more merciful will Christ be with us when we're in Purgatory.
Our Lady asked that we pray very much for the poor souls; that we pray
5 - I believe in God . . .
1 - Hail Holy Queen
1 - Our Father
1 - Hail Mary
1 - Glory Be
1 - Requiem (Eternal rest grant unto them, O Lord, and let perpetual light shine upon them and may they rest in peace. Amen.)
Our Lady said that if we pray the prayers, "we'll deliver so many souls, so many souls!"
Also, there are Prayers for the Souls in Purgatory for each day of the week.