By the goodness of God I was privileged to preach in county Jail and in mens rehabilitation centers for several years as well as gospel missions.
Jail is the place to remove all the slogans, clique and tripe that is passed off as preaching in our churches.
I think of Paul Washer's statement "I am a dying man preaching to dying men".
All the word of God can become an "effective" word, but as earlier posters stated "you need to pray". God quickens us, anoints us...as I am sure you well know.
Go straight for the jugular, expose the old nature and the old man for what it is...let them know in clear unambiguous terms they are there by reason of the sinful nature given free reign to express itself.
Do your best to bring forth scripture designed to crush any hopes they have for self-repair, self-adjustment, attitude changes, straightening-out. If you preach anything that aids in their false hopes you can be certain you will help them commit more crime in the future.
More than likely there will be others that will be much more pleasing to their carnal minds and they may think you harsh or some hell-fire preacher.
The godly preacher must know for certain without a doubt the false teachers will certainly pave the way to hell for them with false hopes and self-help philosophy. They are an enemy to you, make sure they know that.
Set forth the difference between light and dark in the clearest way you can, show from scripture what is good, holy and righteous so that when they repent they know clearly what it is they are do to.
Lastly, a man in prison has a unique temptation to make Jesus a means to his own goals. Being reformed for parole hearings, being 'christian' so the guards and others will take notice and maybe shorten their stay with early release. Don't forget the tried but false "Jesus if you help me I will do thus and thus for you".
Confront directly this temptation as a false path, it is not cross carrying and surrender of self but a saving of self by religious means. This a part of the broad path leading to destruction.
Anyway, think over these things. God has in the past honored his word with the salvation of many men in jail and rehab who turned from darkness to light.
There is only one "way" and youre offering it, everything else is a lie and is designed to keep them in service to Satan and themselves.