I came up with a theological analogy regarding my hair (because all things relate to theology, of course). Hair dye is like the grace of God in the Sacraments, most especially in the Eucharist. Hair represents the soul. Sin prevents the hair dye from penetrating the hair.
For the sake of my analogy, the sin we're dealing with is venial sin, not mortal sin. Mortal sin separates me from God because I choose to turn away from God. In order to commit a mortal sin, three conditions must be met: grave matter, full knowledge, and deliberate consent. Grave matter is a very serious sin against God, like abortion or adultery. Full knowledge means I know what I doing and I know it is a serious sin. Deliberate consent simply means I go and commit the sin anyway. Basically, it's like saying to God, "Up yours!" Sorry, that might come off as a bit brash in my otherwise family-friendly blog, but mortal sin is bad stuff and there's no point beating around the bush.
Venial sin works the way my hair did the other day. It doesn't block the grace of God, but it doesn't allow it through completely either. Venial sins include those "little sins" I commit everyday: swearing, losing my temper, laughing at crass jokes, gossip, etc. Habitual, unrepentant venial sin can lead me to commit moral sin. Or to put it another way, think of your soul like a window. Imagine if you never busted out the Glass Plus and cleaned that window. That's how venial works; the light still gets through, but it's not as brilliant.
Confession is the super-powerful spiritual cleanse that gets my soul right again. The grace of Confession works its way into every part of my soul and allows all the brilliance of God's grace to shine through. Or, in the case of my hair, I went back today and my stylist used super strong hair dye to allow the red highlights to shine through. I've got super cute hair, now to work on getting a super cute soul.