A conclusion should remind readers of the paper’s main idea without dully repeating it. Often the concluding paragraph can be relatively short. By the end of the paper, readers should already understand your main point; your conclusion simply drives it home and, perhaps, leaves readers with something larger to consider. But avoid introducing wholly new ideas at the end of a paper.
Whatever concluding strategy you choose, keep in mind that an effective conclusion is decisive and unapologetic.
Finally, because the conclusion is so closely tied to the rest of the paper in both content and tone, be prepared to rework it (or even replace it) when you revise.
What you can do in a conclusion
Effective conclusion: argument paper (women in the military)
Effective conclusion: argument paper (use of biotechnology in sports)
Effective conclusion: research paper (Internet surveillance in the workplace)
Related topics:
Drafting an introduction
Planning
Revising