In terms of a novel, I would say the basics: don't make the story a monolgue, throw in some action or develop characters so that they come alive, etc. Without those, my writing would be abysmal. So I would say that sticking to the basics is the most valuable writing advice as they provide a base for the entire story.
Whereas with a poem I would have to say: DON'T RHYME. Teachers drilled into my mind that often the best poems do not rhyme; they flow.

Let's share the best of what we've learned about writing.