Return to the thesis that you presented in your introduction and reconsider what you began with. You should have had an open question, a problematic situation or conceptual difficulty that you resolved through your research and analysis in the paper.
Restate, as simply as possible, the main points of your argument. You don't need to repeat every detail of what you've said. The point here is to allow your reader, having just finished reading your essay, to recollect the outline of it in her mind.
Leave the paper open in some way. It's a sign of strength, not weakness, of good research that it not only answers questions but paves the way for future research. Your conclusion is the place to show that this is the case for your argument. For example, maybe your consideration of the use of steam power in the Industrial Revolution opened up questions about investment in manufacturing that readers need to consider.
Resituate the paper in the more abstract context established in your introduction. If you have been examining the role the introduction of steam technology played in the Industrial Revolution, return to the overall framework of the Industrial Revolution now -- hopefully it will look just a little bit different after your argument or analysis.
End your paper with a sentence that sums up what the significance of your research is, within the broader context that you've elucidated. Your reader should hopefully come away from it feeling that he has learned something new, and ending your paper this way will help make sure that happens.