Please pray this prayer daily for the Holy Souls in Purgatory:
"Eternal Rest grant unto them, O Lord, and let Your perpetual light shine upon them. May they rest in peace. And may the Souls of the faithful departed, through the Mercy of God, rest in peace. Amen."
You can also offer this prayer for a specific person by substituting "him" or "them" with a specific name.
Here is a wonderful article by Scott P. Richert about the Catholic Prayer and Purgatory.