Read your work from beginning to end. Having the complete piece in your mind will help you write the conclusion. This is especially important if you've been working on it over a long period of time.
Remind the audience of the main points you made. This can be done by summarizing your main points or restating your thesis. Reminding the audience of your main points will help reinforce them. However, don't restate your thesis or main points using the same words you did in your introduction. Write your thesis in a different way so it seems different to the reader.
Show your reader why your essay is important. This will make it more meaningful. If your reader doesn't think your essay is important, he likely won't remember it.
Give your reader something to take away from the essay. This can be a challenge for her to take action, a question for her to answer about the future or an “aha” statement that speaks to her on an emotional level. Ending your essay with a strong final statement will help the whole essay stick with the reader.
Maintain the tone of your essay from beginning to end. If your essay is more formal, make sure your conclusion is formal as well. If your essay is casual, your conclusion should be more casual. If your conclusion doesn't match the tone of the rest of your essay, it can be jarring to the reader.