D&C 138

Pondering the visit of the Savior to the spirits of men in the spirit world where Peter had taught -

"By which also he went and preached unto the spirits in prison..."  (New Testament | 1 Peter 3:19)

...Joseph F. Smith wondered -

"...how it was possible for him to preach to those spirits and perform the necessary labor among them in so short a time."  (Doctrine and Covenants | Section 138:28)

Being touched by the Spirit -

"...my eyes were opened, and my understanding quickened, and I perceived that the Lord went not in person among the wicked and the disobedient who had rejected the truth, to teach them;
30 But behold, from among the righteous, he organized his forces and appointed messengers, clothed with power and authority, and commissioned them to go forth and carry the light of the gospel to them that were in darkness, even to all the spirits of men; and thus was the gospel preached to the dead.
31 And the chosen messengers went forth to declare the acceptable day of the Lord and proclaim liberty to the captives who were bound, even unto all who would repent of their sins and receive the gospel."  (Doctrine and Covenants | Section 138:29 - 31)

Thus -

"I beheld that the faithful elders of this dispensation, when they depart from mortal life, continue their labors in the preaching of the gospel of repentance and redemption, through the sacrifice of the Only Begotten Son of God, among those who are in darkness and under the bondage of sin in the great world of the spirits of the dead.
58 The dead who repent will be redeemed, through obedience to the ordinances of the house of God..."  (Doctrine and Covenants | Section 138:57 - 58)

As Joseph F. Smith saw the prophets and faithful saints that had previoulsy passed on, he noticed that  -

"...the dead had looked upon the long absence of their spirits from their bodies as a bondage."  (Doctrine and Covenants | Section 138:50)

Obvioulsy having a body of flesh and bones brought great happiness and joy.  And then he learned that -

"These the Lord taught, and gave them power to come forth, after his resurrection from the dead, to enter into his Father's kingdom, there to be crowned with immortality and eternal life,
52 And continue thenceforth their labor as had been promised by the Lord, and be partakers of all blessings which were held in reserve for them that love him."  (Doctrine and Covenants | Section 138:51 - 52)

Contrary to the false doctrine that heaven is laying around and playing harps, we learn that they "continue thenceforth their labor as had been promised by the Lord".  The Prophet Joseph Smith taught -

“When you climb up a ladder, you must begin at the bottom, and ascend step by step, until you arrive at the top; and so it is with the principles of the Gospel—you must begin with the first, and go on until you learn all the principles of exaltation. But it will be a great while after you have passed through the veil before you will have learned them. It is not all to be comprehended in this world; it will be a great work to learn our salvation and exaltation even beyond the grave” (Teachings of the Prophet Joseph Smith, page 348).

It seems eternity will be laboring for the good of other entities. 

On other thought concerning the "faithful elders of the dispensation", what about the sisters?  Joseph F. Smith also taught -

“Now, among all these millions of spirits that have lived on the earth and have passed away, from generation to generation, since the beginning of the world, without the knowledge of the gospel—among them you may count that at least one-half are women. Who is going to preach the gospel to the women? Who is going to carry the testimony of Jesus Christ to the hearts of the women who have passed away without a knowledge of the gospel? Well, to my mind, it is a simple thing. These good sisters who have been set apart, ordained to the work, called to it, authorized by the authority of the holy Priesthood to minister for their sex, in the House of God for the living and for the dead, will be fully authorized and empowered to preach the gospel and minister to the women while the elders and prophets are preaching it to the men. The things we experience here are typical of the things of God and the life beyond us. There is a great similarity between God's purposes as manifested here and his purposes as carried out in his presence and kingdom. Those who are authorized to preach the gospel here and are appointed here to do that work will not be idle after they have passed away, but will continue to exercise the rights that they obtained here under the Priesthood of the Son of God to minister for the salvation of those who have died without a knowledge of the truth” (Gospel Doctrine, page 461).