I do think that reading what great writers have to say about the craft can be inspirational, so here are the "rules" Bradbury, Hemingway and Vonnegut felt were the most important.
1. Write with gusto.
2. In quickness is truth. (Don't think too hard, just write)
3. Write who you are. (Rather than just what you know)
4. Don't write for money or fame.
5. Feed the muse daily.
6. Don't be afraid to explore the attic. (Of the mind)
7. Surprise yourself.
Ernest Hemingway (he had only four, which is no surprise):
1. Use short sentences.
2. Use short first paragraphs.
3. Use vigorous English.
4. Be positive, not negative (it's affordable, not cheap, etc.).
1. Use the time of a total stranger in such a way that he or she will not feel the time was wasted.
2. Give the reader at least one character he or she can root for.
3. Every character should want something, even if it's only a glass of water.
4. Every sentence must do one of two things--reveal character or advance the action.
5. Start as close to the end as possible.
6. Be a sadist. No matter how sweet and innocent your leading characters, make awful things happen to them--in order that the reader may see what they are made of.
7. Write to please just one person. If you open a window and make love to the world, so to speak, your story will get pneumonia.
8. Give your reader as much information as possible as soon as possible. To heck with suspense. Readers should have such complete understanding of what's going on, where and why, that they could finish the story themselves, should cockroaches eat the last few pages.
Vonnegut's are the ones I think I personally identify with the most, and are probably the most practical as far as actual characters and plot may go, thanks to 2, 3, 4, 5 and 6. I think Bradbury's advice not to write for fame and money is essentially the same as Vonnegut's suggestion to write for one person and not the whole world. Bradbury's "write with gusto" and Hemingway's "use vigorous English" are pretty compatible. And I tend to take Vonnegut's last rule to mean make yourself clear and don't hide things unnecessarily from readers, rather than have no suspense at all.
One of my favorite bits of writing advice actually comes from Kurt Vonnegut, who decided to graph the shape of a story. It's a great four minutes on plot that's simple and entertaining. If you have a favorite writer's "rules" or advice to share, add them or link to them in the comments.