I think Steve Kellmeyer does a pretty good job of explaining it all:
skellmeyer.blogspot.com/2008/08/what-must-we-do.html
In particular, for this aspect he says:
Have no attachment to sin, even the most venial. People think this is the most difficult condition, but it isn’t as bad as it sounds. We aren’t expected to be immune from concupiscence. After all, even baptism doesn’t wash away concupiscence, so it would be ridiculous to expect us to have no concupiscence in order to win a plenary. A movement of the flesh to desire something is not the same as attachment to sin. In order to have no attachment to sin, an act of the will is sufficient, e.g., praying, “Lord, I do not desire anything or anyone except Yourself” or “Lord, I reject all sin and all attachment to the things of this world; I cling to the things of heaven, most especially the Beauty and Glory of Yourself.”
I believe Kellmeyer is of the opinion (and I am inclined to agree with him on this point) that God and the Church are not out to set some impossibly high standard such that only the saintliest of saints could ever hope to achieve a plenary indulgence (and only on their best of days). Rather, as our God is a God of infinite mercy, he
wants to give us these graces more than anything.