Examine your topic carefully to determine whether it's broad enough to write a longer paper. Narrow topics tend to limit the amount sources you have at your disposal. You need to have a topic that is fairly broad to begin with, and then narrow it down later on. Try to extend essays on narrow topics is far more difficult than summarizing and cutting out material from an overly verbose one.
Research your topic extensively. Some students run into trouble writing essays because they simply fail to do adequate research. Research needs to be thorough and accurate. A good rule of thumb for any paper at the college level, is one to two sources for each required page of the paper. A 15-page paper, therefore, should have 15 to 30 sources. This means that for each page of your paper, you may only need to provide one paragraph of material from each source.
Ask broad questions about your research. Look for practical implications and considerations related to your topic that you can elaborate on. For instance, if you are writing a paper on cloning, you can approach it from the scientific angle, but you can include additional information that arises from ethical arguments. Professors like to see that you are thinking about the bigger picture.