I have a strong devotion to the souls in Purgatory, and so was quite delighted to find another way of relieving their pain. I found this in The Incredible Catholic Mass:
The suffering souls also experience a great alleviation when the graves are incensed and sprinkled with holy water. The holy water does indeed only materially moisten the earth, but in its beneficent virtue it refreshes the souls in Purgatory, just as the waters of Baptism falling on the head of the infant have the power to cleanse his soul. Therefore see that you frequently sprinkle the last resting place of your friends with holy water, for thereby you will allay the heat of the flames that torture them.
What a simple and easy thing this is to do! And if you wish to apply an indulgence to them, devoutly visit a cemetery and pray for the departed; this indulgence is applicable only to their souls, not for yourself. While you are there, sprinkle as many graves as you can! I have begun the habit of visiting cemeteries that I drive by; just a quick stop to pray a requiem and three Hail Marys for them; now I will add holy water as well.