Your last sentence should be elegant, to the point, and provocative. This is easier said than done. But it all starts with illustrating the point of your essay. Ask yourself What is my essay about, and what am I saying? and then move on from there.
End with a little bit of irony. Be playful with your last sentence and pose an ironic by-product of what you're talking about. Then, the end of your essay becomes especially provocative. Make an appeal to emotions. Much of the time, essays are very rational, forgetting about emotions. That's why appealing to people's emotions can be a really powerful way to conclude an essay. Done in the right way, this will help the article have heart. Just make sure that your conclusion is in keeping with the tone of the rest of your essay. Include a call to action (use sparingly). If your essay is truly about getting people to change, then including a call to action is a useful tool to rouse your base. But use it sparingly: In the wrong context (an expository essay, or an argumentative essay) it can be overkill.