Avoiding and Detecting Plagiarism:  A Guide for Graduate Students and Faculty prepared by the Graduate Center's Office of the Provost and the Office of the VP for Student Affairs is a comprehensive but dated guide to help you avoid plagiarism.

The Grad Center's Writing Center also offers insightful tips for Avoiding Unintended Plagairism.

And the Purdue Online Writing Lab (OWL) offers a thorough guide on avoiding plagairism with practical information for writers and scholars.  It begins:  "Plagiarism is using someone else’s ideas or words without giving them proper credit. Plagiarism can range from unintentional (forgetting to include a source in a bibliography) to intentional (buying a paper online, using another writer’s ideas as your own to make your work sound smarter). Beginning writers and expert writers alike can all plagiarize." (Read the full text of the Avoiding Plagiarism Guide from the OWL.)

Also see the OWL's handy one-page overview that succinctly summarizes when you should cite a source.