Get started early. You won't make as many mistakes or find yourself simply regurgitating what others have written.
Use scholarly sources versus the open Internet. Quality information leads to quality writing, and there is less temptation to recycle unattributed information.
If in doubt, cite the source or contact an instructor or librarian for help.
Recheck your citations and references before handing in your paper to make sure all are complete and accurate.
Use a style manual or a class/library reference tool to check your citation and reference format.
Have a friend who's familiar with your writing look over your paper. Chances are he or she will spot content that doesn't sound like you.
Be original. Your paper is about your ideas and conclusions drawn from research. Reading scholarly writings on your topic is a necessary step, but these should not be the sole content or bulk of your paper.
Compare or contrast ideas from various sources in your paper. Analyze or otherwise contribute to the writings of others with your own input. Do not just repeat or alternate the ideas of others to come up with content.
Summarize a source paragraph in a few sentences instead of trying to translate a single sentence into your own words. If you cannot easily paraphrase in your own words, you may need to quote the desired portion.
Read a passage completely through once or twice before trying to paraphrase or summarize it. Try to recap the essential details without looking at the source so that the words will be your own.
To quote or not to quote? Sometimes an author's exact words make strong impact and should be included for that reason. When language is too technical to easily paraphrase or an author coins a term or phrase, a quotation is a good idea.
Don't cut and paste a direct quote into your document unless you immediately put quotes around it and note the source. It is much harder to confuse your words with another author's when the source is documented immediately.