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One of my professors in college when I was a freshman put it very simply. He said, "In the introduction tell them what you are going to tell them, in the body tell them, and in the conclusion tell them what you told them." So essentially a conclusion is going to rehash what you already stated in the paper, but it is not as simple as it sounds.
In a conclusion you want to summarize the main points of your paper without restating them word for word. Clearly and simply paraphrase your main points bringing them together and focusing them on your thesis (the point your were trying to make). Remember, in a persuasive essay the conclusion is the last opportunity to sway the reader to your point of view. If you tell them what you told them in a way that is too similar to the way that you told them, you will loose them (sorry I was being intentionally verbose there). Find a way to restate your argument without it sounding like you are restating your argument. This take practice and you will write many papers before you get it down.
You should also address the counter argument and briefly state why your viewpoint is the right one. This is important because you do not want the readers that disagree with you to go away forgetting that you make a very good point against their views.
Ending your conclusion with a relevant quote that states the main feeling of your paper can be good. However, writers often use a "call to action" to close their conclusion. A call to action is when you ask the reader to take some kind of action. This could be to stand up for your cause, write their congressmen, or do more research. A call to action is not appropriate for all papers, so carefully consider the context and the class before using one.
Bring up new points or points you did not discuss in the main body of the paper. This is very easy to do because very often we want to put in one last thing. Don't do it. Doing so will definitely affect your grade.