JOEL - Here is the scripture in question:

"And now, verily I say unto you, I, the Lord, will not lay any sin to your charge; go your ways and sin no more; but unto that soul who sinneth shall the former sins return, saith the Lord your God." (D&C 82:7)

In the first part of this scripture God is telling Joseph Smith and other church leaders the same thing He told the woman accused of adultery:

"Woman, where are those thine accusers? hath no man condemned thee?
She said, No man, Lord. And Jesus said unto her, Neither do I condemn thee: go, and sin no more." (John 8:10-11)

So at this point God is forgiving them for sins they have committed in the past(See verses 1-2), with a warning to sin no more.
To understand the second part of this verse consider the following scripture:

"By this ye may know if a man repenteth of his sins-behold, he will confess them and forsake them." (D&C 58: 43).

The true test of whether someone has really repented for a sin is that he will confess and forsake it. So if he commits the same sin again we know that he did not completely repent of it in the first place because he did not completely forsake it. So it's as if he has not yet repented of the previous times he did it.
Brigham Young said:
"It is present salvation and the present influence of the Holy Ghost that we need every day to keep us on saving ground. When an individual refuses to comply with the further requirements of heaven, then the sins he had formerly committed return upon his head [Ezek. 3:20]; his former righteousness departs from him, and is not accounted to him for righteousness: but if he had continued in righteousness and obedience to the requirements of heaven, he is saved all the time through baptism, the laying on of hands, and obeying the commandments of the Lord and all that is required of him by the heavens-the living oracles." (Brigham Young, JD 8:124.)

From the Doctrine and Covenants Commentary:
God forgives the repentant sinner so completely that "his transgressions shall not be mentioned unto him"(Ezk 18:24); on the other hand, the righteous, who turn away from righteousness, cannot hope that his former life will save him. The principle works both ways, and is, therefore, eminently just." (Doctrine and Covenants Commentary Page 490.)

Thus, the effects of repentance are conditioned upon continued righteousness.
So what happens when we do sin again; does that mean we have to repent for all the past sins again? The gospel is not supposed to be difficult. If you were a loving God wanting to have his children come back to you, and giving them every break possible, how would you want your children to understand repentence for repeating sins?
I think we can use the same logic as in D&C 82:7. If sinning again brings back all the previous sins, then it seems to me we can assume that if we repent of the most current violation, it is as if we have again repented of all the former times we sinned, without needing to confess them all again. Our sins can be considered repented for as long as the person continues to forsake them and strives to live a righteous life. This all really depends on what is in our hearts and how sincere we are about repenting and our resolve to not sin again. I think God knows what's in our hearts and if we deserve it He will continue to forgive us according to His judgment.
"I, the Lord, will forgive whom I will forgive, (D&C 64: 10)