The Little Flowers of Saint Francis (a collection of Franciscan lore) recounts an episode when a Franciscan priest (John of Alvernia) offered the Holy Sacrifice of Mass for the souls in Purgatory on All Souls day:
As Brother John was saying Mass on the day after All Saints, for the souls of the dead, as the Church has ordered, he offered with such charity and such compassion the holy sacrifice, which the dead desire above all else we can give them, that he seemed to be overwhelmed and consumed by the ardour of the feelings which filled his heart; and when he lifted up the Body of Christ and devoutly offered it to God the Father, entreating him, for the love of his blessed Son Jesus Christ, who had died on the cross for the souls of men, to deliver from the pains of purgatory the souls of the dead which he had created and redeemed, he saw immediately an immense number of souls go out from purgatory, like innumerable sparks of fire coming out of a burning oven; and he saw them go up to heaven, through the merits of the Passion of Christ, who is daily offered for the living and the dead in that most holy sacrifice, which is worthy to be adored for ever and ever.
This images parallels one given in the third chapter of the book of Wisdom:
 But the souls of the just are in the hand of God, and the torment of death shall not touch them.  In the sight of the unwise they seemed to die: and their departure was taken for misery:  And their going away from us, for utter destruction: but they are in peace.  And though in the sight of men they suffered torments, their hope is full of immortality.  Afflicted in few things, in many they shall be well rewarded: because God hath tried them, and found them worthy of himself.
 As gold in the furnace he hath proved them, and as a victim of a holocaust he hath received them, and in time there shall be respect had to them.  "The just shall shine, and shall run to and fro like sparks among the reeds."  They shall judge nations, and rule over people, and their Lord shall reign for ever.
So pray for the poor souls. If you assist in delivering them from the pains of purgatory, that soul will be a personal saint and intercessor for you!