An artist and lecturer (who I very much admire) has once told me that it took him 20 years to figure out that composition works the very same way on the smaller scale of the mark as it does for the whole picture.
My initial reaction was - "why ofcourse!", but then i started thinking...

That was the single most deceptively simple piece of advise i've ever heard.
(Still thinking 4 years latter. 16 more to go.)