A popular historian, Doris Kearns Goodwin, recently suffered a blow to her credibility by plagiarizing several sources. She claimed the plagiarism was accidental, caused by sloppy note taking.4
The first step to plagiarism avoidance and proper documentation is to cultivate good note taking habits.
Choose a note taking system, such as index cards or a spread sheet program, that will allow you to sort your sources alphabetically for when it comes time to include them in your bibliography.5
For each source you use, keep track of the bibliographic data, including the author, title of the work, title of the publication in which the work appears, name of the publisher, date, and if applicable, number or issue of publication, and the pages you consulted. In short, record whatever information is necessary to properly identify the source consulted.
Annotate each source on your note card or spread sheet. An annotation includes the bibliographic data, plus a brief summary describing the content and your thoughts about how useful the source may be to your research. You should also indicate what you know about the author. Is s/he an expert in the field?6
Distinguish verbatim (word for word) passages from the rest of your notes by setting the words off in "big quotation marks," or bycolor coding them with a highlighter, crayon, or special font color. Also, be sure to note who you're quoting. The author of your source is not necessarily the author of the particular cited idea or phrase you are using.
Notes that are lightly paraphrased could come back to haunt you. Any time you have not sufficiently rephrased something, mark it with a big PM (for "paraphrase more") or color code it.
When you jot down your own ideas, distinguish them from your other notes by writing or typing the word "ME" next to them, in big letters.7
If you've made photocopies or printouts of your sources, or if you have the original sources in hand, save them until you are done writing your paper. That way, you can double check your work against each source to make sure you haven't accidentally plagiarized something.