There is a lot of advice out there for writers. Rules, guidelines, tips. Most of it has some merit, but if you were to follow every little suggestion it would be very constricting and frankly would rob you of a lot of the fun of writing.
But if you were only willing to follow one piece of advice, what would it be? Is there one piece of advice that trumps all others?
The sensible and reasonable answer is that it depends. It depends on what kind of writer you are, what your personal strength and weaknesses happen to be, what your goals are. Each writer is different, each writer may require a different blah, blah, blah.
And that is all very true, but putting all sense and reason to one side, I say yes, there is one piece of advice which if you follow, ignoring all others, will still improve your writing immeasurably (or measurably if you happen to be particularly adept at measuring things).