1. "Show, don't tell"
Just about every writer on earth will have heard this one. It's a great bit of pocket wisdom to pass down to a beginner, but the more I write—and read—the more I realise this advice is to be wielded like a tool. You need to pick the right moments to use it and, more importantly, the right moments not to use it.
Does your reader really need to know about the warm, mushy, bitty texture of the sweet and milky substance in your protagonist's mouth? Or can you get by just letting them know she had oatmeal for breakfast?
My advice to intermediate writers would be to aim for a balance between showing and telling, depending on the pace and tone of your story.
2. "Write every day"
I'm not fond of the implications that tend to come with this advice. That you must force yourself to write, that you'll never "make it" if you don't, that all the great writers did because they had to. These suggestions are often sold with such confidence, it's hard for a new, impressionable writer to question them without feeling silly.
Sure, the more you write, the more opportunities you potentially give yourself to practice your craft. But if you're exhausted or overloaded, how likely is it that you'll use these opportunities well? Wouldn't it be better to take a break and come back stronger the following day? What good are you to anyone if you let yourself get burnt out?
Think of creativity like breathing. There's certainly no shame in inhaling when you need to.
3. "Use proper grammar, spelling and punctuation"
When I first started writing, I was a stickler for this. But as I read more and learned more about how culture shapes language (and vice versa), I stopped worrying so much. First of all, there's no one "proper grammar, spelling and punctuation". Heck, I can name at least 5 same-same-but-different guides on how to write good English.
In fiction, I personally feel the world of your story should determine the grammar, spelling and punctuation you use. In which case, you might not italicise "foreign" words because they might not be foreign to your characters. You might not use American standard English because your characters are Australian. Or you might use more traditional turns of phrase rather than modern ones.
"Proper" is primarily defined by the culture that shapes the language of your story. Presenting it in such a way that your target audience can appreciate, I believe, offers the reader a more immersive and empathy-stoking experience of a different world.