We should always pray for the Holy Souls in Purgatory, but November is the month when the Church specially remembers them. Per the Enchiridion of Indulgences, we can gain a plenary indulgence for the Holy Souls every day for the first eight days of November by praying at a cemetery and fulfilling all the other necessary conditions for a plenary indulgence.
We should get into the habit of praying often, and remembering the Holy Souls whenever we pray. Say the Requiem Aeternam (reproduced below) at the end of your daily Rosary. Pray for the Souls whenever you pass by a cemetery, and when you receive Holy Communion. The Dominican friars have a beautiful custom of reciting the De profundis (Psalm 129) in the cloister walk, where deceased friars were traditionally buried, before meals. The traditional grace after meals -- which we have largely gotten out of the habit of saying, and which is reproduced below -- contains a prayer for the Holy Souls.
The best thing we can do for the Holy Souls is to have Masses offered for them. In this way we can benefit not only the Church Suffering but also the Church Militant, by contributing to the material support of the Church. Your average diocesan priest is basically a beggar: he is obliged to support himself and depends on stipends to supplement his small salary.
The Holy Souls are powerless to help themselves: the time for praying for ourselves and offering up our sufferings and trials ends at death. The Church Suffering is entirely dependent upon the Church Militant and the Church Triumphant for the relief of their sufferings. Can the Souls nevertheless help us in their present state? Opinions differ. The Church does not publicly pray to them for their intercession, but we are permitted to pray privately to them. They are, after all, certain of gaining heaven once they have satisfied all their debts to Divine Justice. It cannot be doubted that, once freed from their prison, they can and do help those who helped them.
Personally, I believe the Holy Souls can help us by their prayers. I once was in dire need of oral surgery at a time when I was out of work and uninsured. In my desperation I appealed to the Holy Souls, and in short order I was connected with an oral surgeon who performed the necessary procedure free of charge. I believe the Souls are at least as anxious to help us as they are to be helped by us -- maybe even more so.
But really, the attention we give to the Holy Souls ought to consist mostly in our prayers and sacrifices for them -- especially those who were our family and friends.
Some prayers for the Holy Souls:
Grace after Meals
+ We give Thee thanks for all Thy benefits, O Almighty God, Who livest and reignest forever; and may the souls of the faithful departed, through the mercy of God, rest in peace. Amen.
V. Requiem aeternam dona ei (eis), Domine.
R. Et lux perpetua luceat ei (eis).
V. Requiescat (-ant) in pace.
V. Eternal rest grant unto him/them, O Lord.
R. And may perpetual light shine up on him/them.
V. May he/they rest in peace.
Prayer of St. Gertrude the Great
Eternal Father, I offer You 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, for those in my own home and in my family. Amen.
And since our primary order of business on this earth is to be worthy to join the Holy Souls someday in eternal beatitude, we should also get into the habit of making provision for our final hour by praying for a holy death. Herewith an appropriate prayer by Bl. John Henry Newman:
O my Lord and Savior, support me in my last hour by the strong arms of Thy Sacraments, and the fragrance of Thy consolations. Let Thy absolving words be said over me, and the holy oil sign and seal me; and let Your own Body be my food, and Thy Blood my sprinkling; and let Thy Mother Mary come to me, and my angel whisper peace to me, and Thy glorious saints and my own dear patrons smile on me, that in and through them all I may die as I desire to live, in Thy Church, in Thy faith, and in Thy love.