You can read the text here. This is the final post in my brief foray into 1 Thessalonians.
The opening verses of this closing section are a little tricky. Who is Paul talking about? Is he thinking of leaders in the church or not? Certainly nothing in the text forces it to refer to leaders. In some senses it's hard to imagine there being formal leadership in place given that Paul had to leave so quickly after founding the church in Thessalonica and that he wrote this letter soon afterward. However, that may be because we're in a different situation where there's never a church comprised solely of new Christians. That question is hard to adjudicate. One wonders how important the question really is when Paul's comments clearly cover anyone performing certain functions.
In any regard, Paul tackles the topic of mutual edification, beginning by urging the Thessalonians to show appreciation for the service of those in the community who teach and care for them. Paul's location outside the community makes this exhortation easier to make. He also is concerned that no one in the community make life difficult on those trying to build up the community, so he encourages the community as a whole to engage in ministry aimed at helping the weak and disorderly become productive and not destructive members of the community. And when one does act destructively, forgiveness and generosity must ensue so that no cycles of evil and unforgiveness are formed.
The community is Paul's focus throughout this passage so we should avoid an unnecessary narrowing of focus in making vv. 16-18 about personal exhortations. Paul is trying to hammer home the need for persistent prayer by and for the community that rejoices in all that God has done for them.
His last major exhortation surrounds prophecy. Clearly from passages like these, 1 Cor 14, and Acts 11 it's clear that prophecy was a common phenomenon in the early church. It is no surprise that there were abuses of it and that Paul felt the need to lay groundwork. Prophecy was a good thing and benefited the community and should not be muzzled. However, after hearing the word it is the responsibility of the community to sift the message keeping the good and discarding the bad.
Paul closes with a blessing and a few final comments as is appropriate given the overall friendly tone of the letter and the previous section. In its essence, the blessing is about the spiritual wholeness of the church, which again is fitting given the previous exhortation. The end result being vindication for those who participate in the holy body when Jesus returns.
Thankfully, Paul requested this letter be read to the whole congregation which must have played a role in its preservation, bringing God's grace not only to the Thessalonians, but to as as well.
 That is Malherbe's way of summarizing verses 12-15.
 Disorderly or disruptive would be a better translation than idle according to Malherbe, Fee, and Gaventa.
 Gaventa makes this point very well.
 Fee draws out very helpfully where the text is addressing the community as a whole and as individuals.