The best-documented advice Frank Herbert ever gave fits in three words: concentrate on story. He wrote it down in his final essay, published the year he died, and credited the idea to his literary agent.

The Direct Answer
Concentrate on story, and let money and success take care of themselves.
Herbert laid this out in his essay for Writers of the Future Volume II (1986), his last piece of writing before his death that February.

The One Rule: Concentrate on Story

Ask what Frank Herbert believed mattered most in writing and the answer is short. Concentrate on story. He called it the single most useful piece of advice he ever received, and he credited it to his literary agent, Lurton Blassingame, who had spent a career watching what made storytellers succeed. Herbert spent the rest of his own career proving it out.

By story, he meant something specific. A good story makes interesting things happen to a character the reader can identify with, someone who grows in stature as the pages turn. The reader keeps going because they want to know what happens to that person. Everything a writer does should serve that pull toward the next line.

A writer's job is to do whatever is necessary to make the reader want to read the next line.

Frank Herbert, Writers of the Future Volume II (1986)

That line is the whole philosophy compressed. The reader's attention is the only currency that counts while you're drafting, and the writer's task is to keep earning it, sentence by sentence. Herbert put it in an essay for a contest he helped judge, and it ran in 1986, the year he died.

Forget About Success While You Work

Herbert's second point grew straight out of the first. Thinking about money or fame while you write, he argued, bleeds attention away from the only thing that can earn them. Do the job on the page and the rest tends to follow. Skip it and nothing you chase will rescue the book. His own history gives the advice some teeth, since Dune collected more than twenty rejections before it went on to sell in the millions.

He also refused to wait on inspiration. Herbert said he never suffered a writing block, and that when he reread his work later he couldn't tell which lines had come easily and which he'd forced out on a stubborn day. On paper they looked the same. The lesson he drew was to sit down and do the work rather than wait to feel ready. For how that discipline played out day to day, see the full Frank Herbert writing routine.

There was a last piece of advice, aimed at the writers coming up behind him. Remember how you learned, he wrote, and when your turn comes, teach. It sat in what turned out to be his final published essay, so it reads a little like a handoff.

What You Can Steal