Learn all the "rules" and know when and why to break them. Over the centuries, those who came before us have established useful and necessary guidelines — these are often called rules or laws and it's important to know them.
Yet, unlike other kinds of laws, it may be acceptable to break them at times so long as you know why. Basic graphic design principles and rules are important and useful to know, yet most professionals today have a hole in their education when it comes to the fundamentals of graphic design. I'll try to do my little bit with
my next book to raise the design mindfulness and vocabulary of professionals who do not make a living in design per se, but who have a desire to get better.
This is not an exhaustive list (in fact, I started with about 25 items); there are many other things designers can teach us (and not only graphic designers as well). What is missing from this list? What would you add? Love to hear your ideas.