A very detailed article about plagiarism from the Georgetown University Honor Council, with plenty of examples of what not to do when writing a paper. Includes an ethical statement and definition related to plagiarism.
The Indiana University Writing Center offers these tips and examples for their students who want to avoid getting into trouble via plagiarism. Covers unacceptable paraphrasing, citation terminology, and strategies for using the Web while avoiding plagiarism. Includes lots of concrete examples.
This brief article from the Online Writing Lab from Purdue University has strategies to help avoid accidental plagiarism.