This Year of Mercy, we are called to perform spiritual and corporal works of mercy. One of the corporal works of mercy is "ransom the captive" or, in a more modern translation, free or visit the imprisoned. Now, for most of us, this is not possible as we don't have prisons handy to drop in and visit prisoners. However, there is one prison full of souls crying out for our mercy, and that is Purgatory.
Souls in Purgatory have no way of helping themselves; they must do their full sentence until they are cleansed of the temporal punishment of their sins. However, we can aid them by our prayers, penance and fasting; pleading with God for their timely release. One most effective help is to offer Masses frequently for the souls, and to offer up your Communion for them. When the priest raises the Chalice during the Mass, offer a precious drop of His Blood to fall on the souls in Purgatory, to wash them clean in the Blood of the Lamb.
This becomes also a spiritual work of mercy, as you are comforting the afflicted and a twofold corporal work of mercy as you care for their thirst for God by offering alms for their release.
Also note that the requiem prayer is indulgenced applicable only to the holy souls, so the more you pray it the more benefit there is for the souls; I like to pray it whenever I pass a roadside cross that marks where someone died, or when I pass a graveyard. It can also be prayed before and after meals and between decades of the rosary. So many opportunities to ransom the captive!
The requiem prayer:
Eternal rest, grant unto them, O Lord.
Let perpetual light shine upon them,
and may they rest in peace. Amen.