Begin your conclusion by restating main points you gave throughout the essay that support your thesis. Use different language than you used in the body or introduction of the essay. When you overuse certain adjectives or nouns, you give a feeling of redundancy and monotony in your words and writing style. Use a thesaurus to help you find different words that have the same or similar meanings.
Write your second-to-last sentence as a creative restatement of your thesis. Again use different language than that of the original statement.
End the conclusion by directly addressing the reader through a question relating to the topic, or by giving a call to action in support of your viewpoint. It's important to engage the reader as much as possible during an argumentative essay, helping her to identify with what you are attempting to convey. Asking the reader a question in your conclusion will serve to further his thinking on the topic. For example, an essay arguing against deforestation might ask, "Would you want the forests, and animals supported by them, in your hometown to be completely eradicated?" or "Could you imagine a landscape devoid of the beautiful greenery--and oxygen--forests provide?"