Look at the research question again. Then, reread your paper. Decide whether the paper answers the question in a focused way, or if it veers off topic. Mark the sections in which it veers off topic, and either delete the sections, or rewrite them and relate them to the original argument.
Decide if your thesis, stated at the end of your introduction, is specific enough, if it truly is in line with what you want to argue, or if you want to make it more nuanced. A thesis statement will often change as the writer does more research and becomes more confirmed in her thoughts and opinions about the paper -- so don't worry about changing your thesis slightly to better fit the paper's body.
Check your introduction to see if it clearly lays out the points that you will be arguing/discussing. Look at it through the eyes of a third party, and determine whether you would be clear on what the paper sets out to accomplish.
Make sure that you spent enough time discussing the salient points, and relatively little time on the trivial points. Writers will occasionally fall into the trap of doing the opposite, because the trivial points are more fun/easy to write about. However, this does not serve your argument. So, if you did spend too much time on the trivial points, rewrite to have a greater focus on the more important ones.
Check your transitions between paragraphs to determine if they flow smoothly. If not, work to make the last sentences of paragraphs lead into the topic sentences of the following paragraphs.
Make sure that you supported, through facts and arguments, the claims that you made in your thesis. If not, you need to "beef up" your paper with more evidence, argument and thought.
Check the facts in your paper. Be sure not to make up facts, as a point of academic integrity. Make sure you cited your sources in the style requested by your instructor.
Re-read your conclusion. Make sure that it wraps up the main points that you sought to make, and if possible, make your paper end on a thought-provoking, stimulating note in which you pose a question to the reader or discuss future directions of your research.
Refine at the sentence level by making your word choices more specific and colorful, making sure that you have variation in sentence structure, writing in the active voice, and combining sentences, where appropriate.