The following prayer will release 1000 Holy Souls from Purgatory each time it is said. It came from St. Gertrude the Great. This would be a good prayer to memorize.
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.