For situations were the string might contain spaces or not, I prefer double quotes and line continuation with backslashes:
key: "String \ with long c\ ontent"But note about the pitfall for the case that a continuation line begins with a space, it needs to be escaped (because it will be stripped away elsewhere):
key: "String\ \ with lon\ g content"If the string contains line breaks, this needs to be written in C style \n.
See also this question.