You have to write one million words of crap before you can write well. This is obviously ridiculous, if taken literally. It isn't like words one through one million are horrible and then, suddenly, like a miracle, you are an incredible writer. But I think the idea behind it makes sense: it takes practice to fulfill your potential as a writer. And, just as with anything else, the more you practice, the better you get.
The road to Hell is paved with adverbs. As Stephen King put it, in On Writing. People love to quote this. Probably because it's a great quote. So, clearly, Stephen King is not a fan of adverbs. But it's not that you can't ever use them. As you can see from this post, I'm sure not opposed to them. But the takeaway from this little pearl of wisdom is, cut the adverbs that don't add anything of significance, and know that you'll probably be taking an axe to a lot of adverbs. Similar advice has been given about other types of words: "filtering" words, for example. ("I heard wind rattling my windows." vs "wind rattled my windows.") Or active voice rather than passive. (The active "She stood in front of me" is better than the passive "She was standing in front of me.") And again, it doesn't mean you can never use filtering words or passive voice. Sometimes they're necessary. Just consider them carefully.
Everything's been done already, just worry about putting a unique spin on it. This is the sort of advice that makes me want to prove it wrong, but, honestly, it's probably true, if you're thinking in an overarching sense. Just look at how many, many, many pages there are on tvtropes. It doesn't mean you can't write about any of these things. That would be impossible. It just means, make your story your own. It's your work, your hours of effort, you want it to be yours. But it also means that you shouldn't worry so much about being unique that you cripple yourself. Other variations on this: write the story you want to write; don't worry about trends. These are great pieces of advice too, in a sense, but at the same time, it's not a good idea to pretend trends aren't happening. They can and do affect what sells, and what agents will be tired of seeing in their query inboxes. It's better if you keep all that in mind going in.
Use "said" instead of descriptive dialogue tags. This is a piece of advice I found alarming, when I first started seeing it. It goes exactly opposite what you learn (or at least what I learned) in school. Teachers always are telling you to find descriptive dialogue tags. Don't have your character just say something, have them say it gleefully, or have them shout it, or snarl it, or sing it, or, well, you get the idea. But in novels, that's not the way to go. As with Stephen King's adverb advice, it isn't that your characters can never say something in a descriptive dialogue tag sort of way. But these should be more sparing. They should mean something. What I've heard is that the word "said" is so plain, it sort of slips through your mind. It does its job and leaves the brain free to focus on the more important things going on in the story. (Example: I love J.K. Rowling and Harry Potter immensely, but I know I am not the only one who was immediately pulled out of the story when, in the fifth book, Ron loudly ejaculates some dialogue. Pulled out enough that I remember it was the fifth book without needing to search. Or maybe this says more about the number of times I've read the books...)
Show, don't tell. Possibly the number one most confusing writing advice in the universe? The idea here is that it's not beneficial to the reader if you tell us something like, "she was a funny person." You can tell us that all you want, but it's better if you show us that by actually having her be funny. Or, showing that your characters are in danger by putting them into a scary situation and having them react appropriately is more engaging than just telling us they were in danger. Of course, you can't show everything about everything in your novel, because that would just be tedious. I think the advice is meant to be kind of a reminder to let your words speak for themselves, and to trust your readers.
Start your story where the action begins. This doesn't mean literally start your story in the middle of the action. At least, it doesn't always mean that. Sometimes the right place to start a story is in the middle of a giant explosion, and sometimes (in most cases, probably) it isn't. But no matter what, you want the place where you start the story to be interesting. To hook readers right away, and not let them go.
Hard work always pays off. So, not to be a downer here near the end of the post, but this is a piece of advice that I honestly find...wrong. I mean, it's encouraging. But. As with anything else, there's a lot more to it than hard work. Which doesn't mean that you shouldn't work hard. You should! Because that's an important piece of it all. It's just that you're setting yourself up to be really disappointed if you expect that your hard work will automatically mean easy querying and submission processes, because this is not always the case. However fast or slow it goes, though, it's still going to feel great when it all starts to come together.
Read everything you possibly can. I don't actually have any commentary on this one, I just felt like it should be included. You should read everything you possibly can. I mean, I assume you wouldn't become a writer if you didn't love books. So don't forget about them!
What are your favorite writerly pearls of wisdom?