Librarians at the University of Texas at Austin
have come  up with a good way to remember
what you need to do to  avoid plagiarizing someone else's words and ideas.

"Use this rule: If you knew a piece of information before you started doing research, generally you do not need to credit it. You also do not need to cite well-known facts, such as dates, which can be found in many encyclopedias. All other information such as quotations, statistics, and ideas should always be cited in your papers."

University of Texas System Digital Library
 (http://tilt.lib.utsystem.edu/module3/citing.htm)