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

Eternal Father, I offer you thee the most precious blood of the 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 own family.  Amen.

Pray For Us St Gertrude the Great