When I was about nine years old, I came across a prayer that "frees 1,000 souls from purgatory each time it is prayed." What a deal! I began to pray it every night, and when I was feeling especially generous, I'd pray it two, three, or even four times, practically seeing the procession of souls going into heaven. I had also heard about Padre Pio, that when the souls of purgatory were freed thanks to his intercession, they would stop by and thank him on their way to heaven. I always had the hope that some day there would be a knock on the door and it would be a soul or two coming to give me thanks.
Remembering that, I can really understand why they speak of a "child-like faith." And yet, it shouldn't be lost. That prayer is one dictated by the Sacred Heart of Jesus to St. Gertrude in the last 1200s, and while it may not free exactly 1,000 souls, why not err on the side of over-trust? The only worry, then, would be to say it in the best conditions possible: state of grace, striving for sanctity... You know, the usual.
Another great way for getting souls out of purgatory is by offering up a plenary indulgence for them. If you're not familiar with that, it's something else very simple that cleans your soul of all temporal punishment. For example, if you stole something and asked for forgiveness, you may be forgiven of your sin, but you still need to return what you stole. Your temporal punishment, if not expiated fully here, is exactly expiated in purgatory. Think of how many souls are going through that right now! But the great thing is, that you can offer up a plenary indulgence for a soul in purgatory, doing him a huge favor. These are the steps for receiving a plenary indulgence:
1. Go to confession 8 days before or after. (You can only receive 1 plenary indulgence a day). You must have no attachment to your sins (truly want to change, and do everything possible not to sin).
2. Receive communion.
3. Pray for the Pope and his intentions.
4. Do something extra (half an hour of adoration, pray the rosary in the church or in family, etc.--You can find lists online).
5. Do all these things with the intention of receiving a plenary indulgence.
It's really quite easy. You get one for yourself and the next day one for your friend. And I say "friend" because that soul in purgatory will be eternally (literally) grateful to you, and as part of that, will be praying for you while he's enjoying heaven.
So give it a shot: say the prayer, get some plenary indulgences, offer up your communions, sufferings, joys, annoyances, etc. so that someone who is really suffering can be freed.
Gotta go, I think I hear a knock at the door.