One simple, but effective prayer for souls is: "O My Jesus, forgive us our sins, save us from the fires of hell, and lead all souls to heaven, especially those most in need of Thy mercy." (from Fatima)

Here's another one from St. Gertrude from Germany in the year 1263. The story goes that she was a Benedictine nun who meditated often on the Passion of Christ, which many times brought tears to her eyes. This was typewritten with the prayer. It says she did many penances and Our Lord appeared to her many times.

It also said Jesus said the following prayer would release 1000 souls from Purgatory each time it was said. Don't know what you will think, but here it is anyways, along with an extension that was added for those living:

"Eternal Father, I offer Thee the most precious Blood of Thy Divine Son, Jesus, in union with the Masses said throughout the world today:
For all the souls in Purgatory
For sinners everywhere
For sinners in the universal church
Those in my own home and within my family. Amen.

St. Faustina's Divine Mercy Chaplet runs along the same lines. Instructions with the prayer can be found at
http://www.ewtn.com/Devotionals/mercy/dmmap.htm