Research your paper topic thoroughly and take extensive notes. One of the primary reasons that students plagiarize is because they simply do not know or understand their paper topic thoroughly enough to write about it intelligently on their own. Instead, they will overly rely on quotes from their sources and may accidentally plagiarize in the process.
Use notecards. As old-fashioned as it may sound in today's computer age, notecards can provide an effective shield against plagiarism if used correctly. All of your notes should be paraphrased. Read your text, close it and then proceed to write down your notes on your notecard. The top line of the notecard should have the author's name and the page number from where you extracted the material. If you have more than one author with the same name or one author with several publications, distinguish between them in a way that will help you avoid confusing them.
Create an outline for your paper once you have completed your research. Use your outline for the basis of writing your paper, not your notecards. Refer back to your notecards as necessary, but don't simply use them to copy from and write your paper. If you have properly digested the work you have been reading, your outline should provide you with sufficient guidance for your paper. Use your notecards to fill in the gaps and provide specific material that supports some of your broader assertions.
Quote authors only when absolutely necessary. The way to determine what is necessary and what is not is to ask yourself whether the quote adds any real value to the paper. Perhaps the author has stated something so profound that a paraphrase will not suffice. In those instances, quote the author and use "quotation marks" around all text that is quoted in verbatim fashion.
Provide footnotes or parenthetical references for all quotes or paraphrases; do not simply quote paragraph after paragraph of material from several different authors. Stringing together the ideas of other authors is not writing a research paper. It is simply copying ideas to the page and connecting them in a longer narrative. This too can be considered plagiarism because the entire sum of your work is the conglomeration of the work of several authors. Any time you quote, you need to provide the correct form of reference as required by your professor and the paper style they choose.
Use plagiarism detection software or websites. Some are better than others. The most well-respected among academics is Turn It In. Check with your instructor or other officials at your school to see if you have free access to this helpful tool. Submitting your paper can help you see where your paper may need some revision to avoid plagiarism.