Research your topic as thoroughly as possible. Many students fall prey to the plagiarism bug because they don't fully understand their topic. This causes them to become overly reliant upon their sources and often borrow from those sources without proper attribution. Understanding your topic will allow you write about it without having to rely upon your sources for information, except to illustrate your main points or provide support for your thesis.
Summarize all of your research before you even attempt to put pen to paper or type a rough draft on your computer. It may create a little extra work for you in the short-term, but it will also help you avoid much more heartache later on. One method for doing this is to buy a set of lined note cards and keep notes on these. Write down the author and page number of the material on the top line of the card. Write a summary of the author's work on the remaining lines of the note card.
Create an outline for your paper based on the major themes of your research. Your paper should have one main theme or idea with several supporting ideas or minor themes that supplement the main theme. Create this outline without using your note cards or any sources.
Write the thesis statement for your paper. This should also be done based only on your reading of your research. Look for a main idea and create a thesis statement based on this theme.
Write the body of the paper based on your outline, not your note cards. Having your note cards organized and handy will be helpful, but you should only use these in your paper if you are looking for specific statistical data or other research that you can use to support the main thesis of the paper. Avoid using the note cards unless absolutely necessary since these are just one step removed from the original source. Conclude your paper with a summary of the rest of your paper. The conclusion should reiterate your main thesis and your supporting statements.
Create your final draft and use plagiarism detection software to ensure that you have not copied any material from a website or other common sources regularly submitted online. Websites like Turnitin.com provide students with a way to avoid plagiarism. Some schools have licensing agreements with the site to help their students avoid plagiarism and help professors detect it. Upload your paper to this or another similar site used by your school. If you do not have access to one of these, you can use a non-academic checker available online like Plagiarisdetect.com or Plagiarismsearch.com.