There is a thing which has been on my mind for some time - the difference between plagiarism and inspiration: why is it that something will be categorized as plagiarism, while others slip through the eye of the needle and are recognized just to have been "inspired"?
When one subscribes to the opinion that not only have most things been said before; most have also been said in a better and clearer way before, then one must simultaneously accept that most of the things one can acheive, must be acheived through inspiration - very little comes from nothing. So the important thing must be to make sure that one has something to be inspired by.
Somewhere, I have seen the quote that you have to read thousands of books, before you can write one, and most likely, this is true, then you will have been exposed to sufficient amounts of information (and quite probably also information of divergent nature and content) to form and formulate your own opinion.
And when you have formed and formulated your own opinion, then you can always start looking back at the things you were inspired by and use these as references, make sure that quotes appear verbatim et cetera - and then, you have not leaned on any single source, not copied, not plagiarized.
(Translated from Plagiat og inspiration, originally published January 18, 2010)