Attachment to venial sin

I've often wondered what it means that one of the conditions for a plenary indulgence is a freedom from attachment to venial sins. It sounds so impossible, considering how often we commit venial sins. St. Francis de Sales addresses this in Introduction to a Devout Life in Part I, Ch. 22. He first says we can't avoid venial sins for any length of time, so that is not what we are talking about (whew!). He then says:

But I tell you that you must purify your soul from all inclination to venial sin;—that is to say, you must not voluntarily retain any deliberate intention of permitting yourself to commit any venial sin whatever.

In other words, what you need to avoid is intentionally reserving for your self permission to commit venial sins. Note: this is sin, not imperfect affections. Those are dealt with in chapter 24.