"Revision" means "to see again." As you already know, however, this new vision is often hard to achieve because we're so much inside of our own writing and thinking process. Since we know what we meant to say, it often looks, to Our eyes, as if we said it. We need to find a way to develop new eyes.
One way to do this is to make a new outline. We know that outlines help us before we write, but they can be equally useful after we've written. Try outlining your paper after you've drafted it. Write down what you take your thesis statement to be. Then go through your paper paragraph by paragraph. Try to sum up the main idea of each paragraph in a word, a phrase, or a sentence.
If you have a hard time doing this summary with a paragraph, it may mean that you're discovering a focus problem. For example, why, in this paragraph about the presidential election of 1968, am I suddenly talking about bananas? Are these two things related? Is that relation clear? If both ideas are important to my thesis, maybe I need to break them up into separate paragraphs. If one idea isn't related to my thesis, then I need to have the courage to get rid of it. True revision requires this kind of courage. Sometimes we need to dump entire paragraphs. Sometimes we may find that our best ideas are buried in one paragraph, and that our other paragraphs are just taking up space. It's hard to get rid of paragraphs we love (they're like old shoes that way), but sometimes it's necessary.
Another way to check for focus problems is to compare your first and last paragraphs. Have you stayed on track? Does your final paragraph contain a clearer statement of your main idea, your thesis, than your introduction? Since we often discover the clearest expression of our ideas as we write, we may find that our final paragraph contains elements that would be useful in the introduction. Or, we may decide that the final paragraph would actually work better as the introduction. Don't be afraid to take big chances, and make big changes, in your revision. It may be scary at first, but you'll soon discover that this kind of large-scale revision is actually liberating.