You often hear people say, "It's not deliberate plagiarism, just carelessness." Actually, however, cross-contamination is a better metaphor for most cases of plagiarism than theft. The idea that plagiarism is stealing ideas or words is confusing for a lot of people, because theft is usually a deliberate act. "I didn't mean to do it" is not a legitimate excuse, and non-intentional plagiarism is still plagiarism. Imagine I got on a bike, thinking it was mine, and got home with it. Later I realize it is not mine. I didn't mean to steal the bike, but I still did something wrong.
[Of course, wholesale appropriation of others' ideas is theft, and is plagiarism.]
So the secret to avoiding plagiarism is not to be careless. It is a matter of higiene: keeping your words separate from those of others, meticulously keeping track of who said what / when / how.
I'm assuming most food poisoning occurs not because someone is trying to poison the food, but because of a lack of hygiene. You can have separate notebooks for quotes from other people and your own ideas, just like you might have separate chopping boards for vegetables and meat. You should never transcribe a quote into your document without quotation marks or indentations. Don't go back and format later, because you might not remember. You should have a protocol in place to protect your scholarly integrity.