Today we talked about "justification" and "sanctification" through the grace of the Lord (Doctrine & Covenants 20:29-32). Since one of my favorite scriptures is "... we know that it is by grace that we are saved, after all we can do." (The Book of Mormon, 2 Nephi 25:23) I thought, how can I do "all I can do" in order to attain justification and sanctification?

Could do you explain the expression "all we can do"?

It is my understanding that Nephi's expression "all we can do" is equivalent to the contemporary expression in the English language "doing our best."

In this telestial or fallen world, full of ambiguities generated by the complex interactions of multiple imperfect variables, one will not reach the point of being 100% error-free.  But we can demonstrate our desire to reach perfection by using the best knowledge and common-sense in making decisions.

Perhaps here it might be good to remember the words of Presidents Brigham Young and Lorenzo Snow about reaching perfection in this life.

"It may appear strange to some of you, and it certainly does to the world, to say it is possible for a man or woman to become perfect on this earth. ... If the ... passage ... is not worded to our understanding, we can alter the phraseology of the sentence, and say, 'Be ye as perfect as ye can,' for that is all we can do, though it is written, be ye perfect as your Father who is in heaven is perfect. He cannot be any more perfect than he knows how, any more than we. When we are doing as well as we know how in the sphere and station which we occupy here, we are justified in the justice, righteousness, mercy, and judgment that go before the Lord of heaven and earth. We are as justified as the angels who are before the throne of God. The sin that will cleave to all the posterity of Adam and Eve is, that they have not done as well as they knew how." (Discourses of Brigham Young, p.89. December 18, 1853)

Speaking about recent converts from Europe who emigrated to the United States in the early days of the Church, President Lorenzo Snow stated:

"... they were as perfect as men could be under the circumstances, and in the sphere in which they were acting; not that they were perfect in knowledge or power, etc., but in their feelings, in their integrity, motives, and determination. And while they were crossing the great deep, providing they did not murmur nor complain, but obeyed the counsels which were given them, and in every way comported themselves in a becoming manner, they were as perfect as God required them to be. ...

"A person may be perfect in regard to some things and not others. ... if we could read in detail the life of Abraham, or the lives of other great and holy men, we would doubtless find that their efforts to be righteous were not always crowned with success. Hence we should not be discouraged if we should be overcome in a weak moment; but, on the contrary, straightway repent of the error or the wrong we may have committed, and as far as possible repair it, and then seek to God for renewed strength to go on and do better." (Teachings of Lorenzo Snow, pp.32-33. April 7, 1879.)