Bring your essay full circle by restating an idea first mentioned in the introduction. Revisit, for example, a quote you started the essay with but now with new understanding that your essay's body helped to unravel.
Restate your thesis by briefly reiterating the main points that you used to prove it within the body of the essay. Resist the urge to simply summarize the main points of your essay. Instead, focus on completing your argument to its fullest by presenting the points in a different way -- one that naturally presents a different facet of the gem of your argument.
Answer any questions that have been left unanswered in your introduction or the main body of the essay. The conclusion is your last chance to tie up any loose ends. Smooth out any wrinkles in your argument so as not to leave the audience with any concerns.