I call the end of a conclusion the “concluding thought” because it is the last thing you leave your reader with.  You want to include something that is going to leave your reader feeling satisfied.  One idea is to include a “call to action” asking your reader to do something.  If your essay is on global warming, you ask your reader to drive less, and so on.  Another idea is to include a quote and then explain how the quote relates to the theme of your essay.  This works well for history.  Find a meaningful quote related to your issue.  However, this does not mean you cannot have a “call to action” in a history essay.  For example, if you were writing about the Holocaust, the “call to action” could be about tolerance.

Sometimes the conclusion differs very little from the introduction. The important thing is to make sure that you have clearly stated just what it is you wanted the reader to get out of your work. In reality, the introduction and the conclusion are the two sections of a paper, or even a scholarly book, that most people read. So be sure that you restate just what it is that you've said in your paper.

You should be referring back to your overall main idea (thesis) that should have been stated in your introduction. Don't bring up new support and don't simply repeat what you have already said, and for heaven's sake don't write "and that's the end of my essay" or anything remotely like that.

The precise nature of your conclusion depends on the kind of essay you are writing. For example, if you are writing a persuasive paper, it's effective to end with a "call to action" in which you are attempting to get someone to do something.

The conclusion should draw the body paragraphs back together again.  After the introduction of the thesis in the first paragraph, your paper should have branched out to cover the topic.  The concluding paragraph should pull it all back together again.  This isn't a time to bring up new material, but rather to show how the material you have covered fits together.  The idea is to leave the reader with a concise idea or point.  Think of it like the bow on a package.  The bow helps hold the wrapping paper in place and gives the gift a more polished look.  Once you have wrapped up your argument, you can try to end your conclusion with a punch.  Introductions start with a hook to draw the reader in and a good conclusion will end with a final kick that leaves the reader with something to think about.

Basically, you need to do a very brief recapitulation of the main points of your argument in the body of your essay.  Don't bring in new information, just go over what you already said so that you leave the reader with your main argument in their mind.