The rules for plagiarism are the same as they are for any other discipline: it takes hard work, talent and perseverence to plagiarize successfully. And as with all endeavors, if one is going to plagiarize, one may as well do it right.
The first step to plagiarizing successfully is to take stock of common sense rules. If, for example, you intend to plagiarize for your course on, say, Medieval Europe, it makes sense that the paper you intend to filch be on Medieval Europe. However, Medieval Europe, like most worthy paper topics, is a vast field comprising many sub-fields, some of which are interdisciplinary. There’s the Catholic Church, the Inquisition, fiefdom, Richard the Lion-Hearted, Gothic architecture, St. Augustine, Averroes, Spanish jarchas, the Crusades, Andalusia, the humours, the Magna Carta, Hildegard von Bingen, the troubadours, taxation, Saladin, the plague, Christopher Columbus, King John, Eleanor of Aquitaine, courtly love poetry and chivalric romance, Lancelot, Sir Gawain and the Green Knight, The Song of the Rood, mystery plays (The Second Shepherd’s Play is an especially good one–very funny, involves hiding a sheep in a baby cradle), Christine de Pisan, Marie de France (never could figure out why people don’t know who she is–it’s not definitive, of course, but the evidence seems to suggest that she’s the daughter of Louis the Seventh (is it the Seventh? well, anyway, whoever the Louis was who was married to that patroness of arts, the duchess of Provence–and whom she later divorced–amazing that people could divorce back then)).
Anyway, the point is, how would you know what paper to plagiarize? It would be a hell of a lot more trying to figure out which topic is closest to what you need to write on than to just do the damn work in the first place.