We know that plagiarism is to be avoided at all costs. But first, what is it exactly? The Oxford English Dictionary defines plagiarism as "The taking and using as one's own the thoughts, writings, or inventions of another." Aside from being against copyright laws, plagiarism can destroy a freelance writer's career. Effective ways to guard against plagiarism are don't reword, don't rewrite, and clearly attribute the original writer when using or incorporating someone else's words or ideas.
Research, Research, Research
The more research you conduct on your subject, the more familiar you become with the topic you are writing about. Part of the research process is making notes: this often takes the form of copy-and-pasting, summarizing paragraphs, and paraphrasing sections of text.
Establish your own system or process to separate the research you have gathered from your original draft. For example, I create a separate research document for each article I write; if I copy-and-paste specific information pertinent to what I'm writing directly into the draft document, I'll highlight the text to remind me that I did not write it.
Two-Word Rule
You've found a great blog or some reference materials that perfectly describe the topic you are writing about. Transform the text into your own words. If you are reciting verbatim more than two words consecutively, you are technically paraphrasing without properly citing the source.
Citing Sources
When "Dogs are scared of thunder" that is common knowledge and doesn't need any citation. However, if "...55% of dogs in North America are scared of thunder" that is a specific fact and will need to be properly cited to avoid plagiarism.
Detection Software
Unfortunately, accidental plagiarism can be just as damaging to a writer's career as intentional plagiarism. If you're writing online copy or a print article, use a plagiarism checker app or program. There are some free ones available. Whether it's a 2000-word proposal, one paragraph you're unsure of, or sections of a client's website, running text through a plagiarism detection software program could save your business and protect your reputation.