We can aid the poor souls, not only by offering prayers, but by other acts of virtue: by almsgiving, by accepting a cross. Let us remember particularly the souls most abandoned, who are sometimes the most holy.
God is pleased to reward our least service. And these souls, too, will not fail to aid us by their own gratitude in heaven. Even before their deliverance they pray for all benefactors. They have charity, which indeed excludes no one but which imposes on them a special duty toward those friends. Their prayers are efficacious even if they do not know in detail our condition, just as our prayers for them are efficacious though we do not know their condition.
--Reginald Garrigou-Lagrange, Life Everlasting