Our Lord told St. Gertrude the
Great that the following prayer
would release 1000 Souls from
Purgatory each time that it is said.
The prayer was extended to
include living sinners as well:

"Eternal Father, I offer Thee the most Precious Blood of Thy Divine Son, Jesus, in union with all the Holy Sacrifices of the  Mass celebrated throughout the world today, for  the Holy Souls in purgatory, for sinners everywhere, for sinners in the Universal Church, those in my own home and within my family. Amen".