Mosiah Chapter 4

4:2-3 - "...and they all cried aloud with one voice, saying: O have mercy, and apply the atoning blood of Christ..."
Why apply the atoning blood? The verse explains:
"That we may..."
1) Receive forgiveness of our sins
2) Our hearts may be purified
3) Spirit of the Lord come upon us
4) Be filled with joy
5) Have peace of conscience
This is how we can apply the atonement - by having faith, repenting, and letting virtue garnish our thoughts unceasingly.  Then we gain these amazing blessings.  We should be applying the atonement daily.

Elder David A. Bednar talked about this in the November 2007 Ensign, Pg. 80-83.  He talked about clean hands and a pure heart.
"In this verse we find the twofold blessing of both forgiveness of sin, suggesting clean hands, and the transformation of our nature, signifying pure hearts...brothers and sisters, it is possible for us to have clean hands but not have a pure heart.  Please notice that both clean hands and a pure heart are required to ascend into the hill of the Lord and to stand in His holy place."
I don't think I could put it better.  Let's do our best to apply this.

4:11-12 - "...Remember...the greatness of God, and your own nothingness, and his goodness and long-suffering towards you, unworthy creatures, steadfastly in the faith of that which is to come..."
If we do this, we shall always rejoice, and be filled with the love of God, and always retain a remission of our sins...and we shall grow in the knowledge of the glory of him that created us, or in the knowledge of that which is just and true...as the verses specify.
We want to retain this remission of our sins, and it takes work.  It takes a change of heart.  But we can receive it.  And what a blessing!

4:20-25 - "...he has [not] suffered that ye have begged in vain...he has poured out his Spirit upon you...o then, how ye ought to impart of the substance that ye have one to another..."
We need to understand that God grants according to our petitions, and we need to become like God.  We need to be careful when we judge what people do and don't deserve.  It's interesting, because often I don't want to hand out money to people on the street...probably because I don't want them to be tempted and abuse the gift.  However, in certain situations, such an act *could* technically be the catalyst that helps them change their life around.  That's why it's important to follow the Spirit and be ready to do what He asks us to do in any situation.